Stylus sensing circuit and stylus

Abstract

A stylus sensing circuit includes a receiving circuit, a demodulation circuit, and a detection circuit. The receiving circuit receives a downlink signal from a stylus, the downlink signal comprising a plurality of frequency signals. The demodulation circuit, coupled to the receiving circuit, demodulates the downlink signal to extract each frequency signal from the plurality of frequency signals. The detection circuit, coupled to the demodulation circuit, selects one of the plurality of frequency signals for signal processing.

Description

Stylus sensing circuit and stylus This invention relates to a stylus sensing circuit and a stylus, and more particularly to a stylus sensing circuit and a stylus capable of multi-frequency transmission. An active stylus is a common peripheral device used in electronic devices with touch panels, such as mobile phones or laptops. Most active styluses on the market are bidirectional, receiving uplink signals from the touch panel and sending downlink signals back to it to perform stylus control. To reduce stylus costs, the industry has developed a low-cost unidirectional stylus (such as the easy pen). A unidirectional stylus only has one-way transmission capability with the touch panel; that is, it is allowed to continuously send downlink signals to the touch panel without any uplink synchronization. For bidirectional styluses, if the touch panel determines that there may be noise interference at a specific frequency of one of the downlink signals output by the stylus, it can notify the stylus via the uplink signal to instruct the stylus to change its output frequency. However, unidirectional styluses cannot receive uplink signals, and therefore cannot change their output frequency under noise interference, resulting in poor noise immunity. Therefore, the main objective of this invention is to provide a novel noise-resistant stylus sensing circuit and stylus to solve the above-mentioned problems. One embodiment of the present invention discloses a stylus sensing circuit, which includes a receiving circuit, a demodulation circuit, and a detection circuit. The receiving circuit receives a downlink signal from a stylus, the downlink signal comprising a plurality of frequency signals. The demodulation circuit is coupled to the receiving circuit and demodulates the downlink signal to extract each of the plurality of frequency signals. The detection circuit is coupled to the demodulation circuit and selects one of the plurality of frequency signals for signal processing. Another embodiment of the present invention discloses a stylus comprising a plurality of signal generators and a mixer. The plurality of signal generators are used to generate a plurality of frequency signals with different frequencies. The mixer is coupled to the plurality of signal generators and is used to combine the plurality of frequency signals to generate a downlink signal. A first frequency signal and a second frequency signal among the plurality of frequency signals carry the same information. As mentioned above, conventional unidirectional styluses (such as easy pens) have poor noise immunity because they can only output a single frequency downlink signal and the frequency cannot be changed. If noise occurs at the frequency of this downlink signal, the output signal is easily interfered with. To address this issue, this invention proposes a novel stylus that can output downlink signals containing more than two frequency components. If any frequency component is subject to noise interference, the stylus sensing circuit of the touch panel can still process the downlink signal at other frequencies to obtain accurate sensing results free from noise interference. Figure 1 is a schematic diagram of a stylus 10 according to an embodiment of the present invention. The stylus 10 includes a plurality of signal generators 102_1 to 102_n, a mixer 104, and a digital-to-analog converter (DAC) 106. As shown in Figure 1, a plurality of signal generators 102_1 to 102_n can generate and output frequency signals F1 to Fn with different frequencies, where n can be any integer greater than or equal to 2. Each frequency signal F1 to Fn generated can be a single-tone signal, which can be a sine wave signal or a square wave signal with a single frequency, and the type of waveform is not limited to these. Mixer 104 is coupled between signal generators 102_1 to 102_n and digital-to-analog converter 106, and can be used to combine frequency signals F1 to Fn to generate a downlink signal V_DL. Then, digital-to-analog converter 106 converts the digital downlink signal V_DL into analog form and outputs the downlink signal V_DL. Figure 2 is a schematic diagram of a stylus sensing circuit 20 according to an embodiment of the present invention. The stylus sensing circuit 20 includes a receiving circuit 200, a demodulation circuit 210, and a detection circuit 220. As shown in Figure 2, the receiving circuit 200 can be used to receive a downlink signal V_DL containing frequency signals F1 to Fn from the stylus 10, and includes an analog front-end (AFE) circuit 202 and an analog-to-digital converter (ADC) 204. The analog front-end circuit 202 may include a sample-and-hold circuit for sampling the downlink signal V_DL. The analog-to-digital converter 204 then converts the analog downlink signal V_DL (such as a voltage signal) into digital form and outputs the downlink signal V_DL to the demodulation circuit 210. The demodulation circuit 210 is coupled between the receiving circuit 200 and the detection circuit 220, and can be used to demodulate the downlink signal V_DL to extract each frequency signal F1 to Fn. For example, if the downlink signal V_DL contains two frequency signals F1 and F2, the demodulation circuit 210 can recover the frequency signals F1 and F2 through demodulation. The frequency signals F1 and F2 can be single-frequency signals, equivalent to the frequency signals generated by the signal generator of the stylus 10. In one embodiment, the demodulation circuit 210 can perform convolution on the downlink signal V_DL using a reference signal corresponding to a first frequency to generate the frequency signal F1, and perform convolution on the downlink signal V_DL using another reference signal corresponding to a second frequency to generate the frequency signal F2. The generated frequency signals F1 to Fn can be further transmitted to the detection circuit 220, which can select one of the frequency signals F1 to Fn for signal processing. In one embodiment, the detection circuit 220 can determine which frequency is affected by noise interference and select the frequency signal F1 to Fn that is not affected by any noise interference for subsequent signal processing. For example, as shown in Figure 2, suppose the downlink signal V_DL contains two frequency signals F1 and F2. The frequency of F1 is affected by noise V_N and is received by the stylus sensing circuit 20 along with the downlink signal V_DL. The detection circuit 220 can then determine that the frequency signal F1 is affected by noise V_N and select the frequency signal F2 for subsequent signal processing. The purpose of this multi-frequency implementation is to reduce or eliminate the impact of noise interference from the downlink signal V_DL on stylus operation. Noise V_N may occur randomly and is unpredictable, thus it may interfere with any frequency signal at any time. In this case, the stylus sensing circuit 20 (i.e., the detection circuit 220) can select any other frequency signal unaffected by noise interference for signal processing. In this situation, the necessary information to be provided by the stylus 10 should be contained in each frequency signal F1 to Fn; therefore, regardless of which frequency signal is selected, the detection circuit 220 and its back-end circuitry can obtain the necessary information. In one embodiment, each frequency signal among F1 to Fn can carry the same information; therefore, the stylus sensing circuit 20 can select any frequency signal F1 to Fn for signal processing and successfully obtain this information. For example, the stylus 10 can be requested to transmit pressure information to the stylus sensing circuit 20, and the frequency signals F1 to Fn contained in the same downlink signal V_DL can carry the same portion of the pressure information. For example, the frequency signals F1 to Fn can contain the same data code and / or the same signal strength, so that the detection circuit 220 can extract the same information after performing corresponding signal processing. Based on the noise detection results, regardless of which frequency signal is selected, the detection circuit 220 can obtain all the necessary information. In another embodiment, the frequency signals included in the downlink signal can carry different information. The stylus sensing circuit 20 can select one or more suitable frequency signals for signal processing to achieve noise reduction. Furthermore, by combining multiple frequency signals in the downlink signal, the signal bandwidth used to transmit the required downlink information can be increased, thereby reducing the downlink information transmission time. For example, stylus 10 is required to transmit pressure information to stylus sensing circuit 20, as shown in Figure 3. The pressure information can be divided into four parts, namely pressure information PI1 to PI4. Pressure information PI1 and PI2 can be contained in a downstream signal V_DL1, and pressure information PI3 and PI4 can be contained in another downstream signal V_DL2. Each downstream signal V_DL1 and V_DL2 can contain two frequency signals F1 and F2. In this example, frequency signal F1 in downstream signal V_DL1 is used to carry pressure information PI1, frequency signal F2 in downstream signal V_DL1 is used to carry pressure information PI2, frequency signal F1 in downstream signal V_DL2 is used to carry pressure information PI3, and frequency signal F2 in downstream signal V_DL2 is used to carry pressure information PI4. When the stylus sensing circuit 20 receives the downlink signal V_DL1, it can determine whether the frequency signal F1 or F2 is affected by noise interference, and select either F1 or F2, or both, for signal processing, thereby demodulating to obtain pressure information PI1 and / or PI2. When the stylus sensing circuit 20 receives the downlink signal V_DL2, it can determine whether the frequency signal F1 or F2 is affected by noise interference, and select either F1 or F2, or both, for signal processing, thereby demodulating to obtain pressure information PI3 and / or PI4. Assuming that the stylus 10 has the ability to receive uplink signals or can receive instructions from the stylus sensing circuit 20 through other interfaces, if any pressure information PI1 to PI4 cannot be successfully obtained due to noise interference or any other problem, the stylus sensing circuit 20 can send a notification to the stylus 10 to request retransmission. In addition, the stylus sensing circuit 20 can also reconstruct complete pressure information based on the received partial information. In fact, depending on system requirements, different frequency signals contained in the same downlink signal can carry the same or different information to improve the flexibility of information allocation in the downlink signal. For example, a downlink signal can contain three frequency signals F1 to F3, and the first frequency signal F1 and the second frequency signal F2 can carry the same information (or the same part of the downlink information), while the third frequency signal F3 can carry different information (or different parts) from the first frequency signal F1 and the second frequency signal F2. It is worth noting that the stylus sensing circuit usually receives downlink signals through voltage changes caused by capacitive coupling on the touch panel. The resulting voltage changes are extremely small and are easily affected by noise. This noise may include display noise caused by load changes due to changes in display data, charging noise from any power source, or any other environmental noise, but is not limited to these. It should also be noted that the circuit structure of the stylus sensing circuit 20 is only one exemplary embodiment of the present invention. In another embodiment, the demodulation circuit may include more than two demodulators. For example, suppose a demodulator can be used to demodulate four frequencies, and the received downlink signal contains eight frequency signals. In this case, the demodulation circuit may include two demodulators to process the downlink signal. In one embodiment, the stylus sensing circuit 20 may further include a memory (not shown), such as static random access memory (SRAM), which may be coupled between the demodulation circuit 210 and the detection circuit 220 to store the raw data of frequency signals F1, F2, etc., obtained through the demodulation operation. The raw data may include signal strength information received at a portion of the touch panel. The detection circuit 220 may use the raw data of the selected frequency signals to calculate the signal distribution of the stylus, thereby obtaining the coordinates of the stylus on the touch panel, wherein the selected frequency signal is located at a frequency determined to be free from noise interference. In one embodiment, the stylus sensing circuit 20 may further include a filter (not shown) that can be used to filter out some unnecessary frequencies that do not carry any frequency signals so that the demodulation circuit 210 can extract the desired frequency signals. Figure 4A illustrates stylus detection performed by a stylus sensing circuit according to an embodiment of the present invention. The stylus sensing circuit (such as stylus sensing circuit 20 in Figure 2) can be integrated with a touch sensing circuit having finger touch (or hand touch) functionality to operate in a pure hand mode or a stylus mode. When stylus sensing circuit 20 operates in pure hand mode, stylus detection and finger touch detection are time-division multiplexed. More specifically, as shown in Figure 4A, a portion of the time slot is allocated for stylus detection, and a portion of the time slot is allocated for finger touch (or hand touch) detection. A time slot used for stylus detection can be allocated to a row or a column of sensor pads, but is not limited thereto. Assume that the downlink signal V_DL expected to be output by the stylus 10 contains two frequency signals F1 and F2. During the time slot used for stylus detection, the detection circuit 220 can detect the frequencies F1 and F2 to determine whether a frequency signal exists at the detected frequency. In one embodiment, a threshold value TH can be set so that the detection circuit 220 can determine whether the signal strength at the detected frequency is greater than the threshold value TH, thereby determining whether the stylus 10 exists. In one embodiment, when the detection circuit 220 detects that the signal strength of each frequency signal is greater than the threshold value TH, it can determine that a stylus is approaching or contacting the touch panel and output a downlink signal V_DL, thereby controlling the stylus sensing circuit 20 to switch to stylus mode. For example, as shown in Figure 4A, the downlink signal V_DL includes two frequency signals F1 and F2. In one time slot, the detection circuit 220 detects that the strength of both frequency signals F1 and F2 is greater than the threshold value TH, indicating that a stylus is approaching or contacting the touch panel. In this case, the detection circuit 220 can determine that a stylus has been detected and control the stylus sensing circuit 20 to enter stylus mode to perform further operations, such as stylus positioning. It is worth noting that the downlink signal V_DL is composed of multiple frequency signals F1, F2, etc. Therefore, a stylus is only detected when each frequency signal contained in the downlink signal V_DL is detected. Thus, if no frequency signal expected to be received in the downlink signal V_DL is detected, it means that the signal strength measured at at least one frequency signal is less than the threshold value TH. In this case, the detection circuit 220 does not determine that a stylus is present, and thus controls the stylus sensing circuit 20 to remain in the pure hand mode. In conventional stylus control systems, the downlink signal is transmitted via a single frequency. If this frequency is affected by noise interference, causing the received signal strength to exceed a certain threshold, the stylus sensing circuit in pure hand mode will incorrectly enter stylus mode. In contrast, in this invention, the downlink signal consists of multiple frequency signals, and the stylus sensing circuit determines whether to enter stylus mode based on the detection of all frequency signals. This ensures that the stylus sensing circuit enters stylus mode at the correct time and eliminates noise interference. When the stylus sensing circuit 20 is in pure hand mode, noise detection can be performed in a similar manner. By comparing with the threshold TH, the detection circuit 220 can detect the noise corresponding to each frequency signal, that is, the noise that may interfere with the frequency signals carried in the downlink signal V_DL. Based on the noise detection results, the detection circuit 220 can select a frequency signal that is not affected by any noise interference for subsequent signal processing. Figure 4B illustrates noise detection performed by the stylus sensing circuit according to an embodiment of the present invention. In a time slot, no stylus is near or in contact with the touch panel, but finger touch on the touch panel is interfered with by charging noise, which has a noise component located at a first frequency of one of the frequency signals F1. Therefore, the detection circuit 220 can detect that the signal strength at the first frequency is greater than a threshold value TH, while the signal strength at a second frequency of one of the frequency signals F2 is less than the threshold value TH. In this case, the detection circuit 220 can determine that noise exists at the first frequency. Therefore, when performing stylus detection in a subsequent time slot, the detection circuit 220 can select a clean second frequency that is not affected by noise to perform relevant signal processing and calculation. In one embodiment, the stylus sensing circuit 20 can simultaneously perform stylus detection and noise detection. As described above, the stylus sensing circuit 20 in pure hand mode can perform stylus detection and noise detection in a similar manner by comparing the signal strength received at a specific frequency with a threshold value. Therefore, based on the detection results regarding the signal strength at each frequency, the detection circuit 220 can determine whether a stylus is present and whether noise exists at any of the detected frequencies. When the stylus sensing circuit is in stylus mode, the detection circuit 220 can select an optimal frequency for signal processing based on the signal variation of the frequency signal. If a stylus is detected approaching or touching the touch panel, the stylus sensing circuit 20 can enter stylus mode, allowing the detection circuit 220 to obtain more signal information for frequency signal selection. It should be noted that if a frequency has a large signal variation, this frequency is more likely to be affected by noise interference; if a frequency has a small signal variation, this frequency may be more suitable for subsequent signal processing and calculation to avoid noise interference. Figure 5 illustrates frequency selection in stylus mode according to an embodiment of the present invention. In this example, the downlink signal V_DL may include three frequency signals F1 to F3. The stylus sensing circuit 20 can receive the downlink signal V_DL during a series of frame periods FP1 to FP6, and the demodulation circuit 210 can demodulate the downlink signal V_DL to extract the signal strength of the frequency signals F1 to F3 during frame periods FP1 to FP6. The signal strength can be further normalized to the same level for comparison, and the relevant data for each frequency can be obtained through demodulation and then stored in memory. Figure 5 illustrates the normalized signal strengths of frequency signals F1 to F3 within frames FP1 to FP6. Based on the changes in these signal strengths, the detection circuit 220 can select an optimal frequency signal for signal processing. Preferably, the detection circuit 220 can select the frequency signal with the smallest signal change for signal processing. In this example, the signal change amount (VAR) can represent the maximum signal value minus the minimum signal value. As shown in Figure 5, the signal change amount VAR for F1 is 100, for F2 it is 50, and for F3 it is 400. Therefore, the frequency signal F2 with the smallest signal change amount can be selected. It is worth noting that in actual circuit systems, each frequency signal F1 to F3 received by the stylus sensing circuit 20 exhibits some degree of variation. Therefore, the detection circuit 220 should preferably select a single frequency signal for subsequent signal processing to minimize noise interference. The selected frequency signal can be the one with the smallest signal variation and is determined to be the least affected by noise. In one embodiment, suppose the detection circuit 220 is currently using frequency signal F3 to calculate the coordinates of the stylus 10, and noise may occur at the frequency of signal F3. Due to noise interference, after receiving frequency signals F1 to F3 during frame periods FP1 to FP6, the detection circuit 220 can determine that the signal change of frequency signal F3 is equal to 400, which is greater than the threshold used to trigger frequency hopping. In this case, the detection circuit 220 can perform frequency hopping to use another frequency for signal processing and to calculate the stylus coordinates. In this example, since frequency signal F2 has the minimum signal change, the detection circuit 220 can switch to using frequency signal F2 during frequency hopping. In another embodiment, when frequency hopping is triggered, the target frequency signal can be selected in a different manner. For example, the detection circuit 220 can select a frequency signal that was determined to be free from noise interference in a previous noise detection operation, such as the noise detection performed in manual mode as described above. Figure 6 illustrates the integration of stylus detection and noise detection based on the downlink signal V_DL, which can be output from stylus 10 to stylus sensing circuit 20. As shown in Figure 6, the downlink signal V_DL includes two frequency signals F1 and F2. Assuming that stylus sensing circuit 20 was originally operating in pure hand mode, based on the received downlink signal V_DL, detection circuit 220 can determine whether stylus sensing circuit 20 needs to switch to stylus mode. At time T1, frequency signal F1 experiences drastic signal changes due to noise interference. Therefore, detection circuit 220 determines that the signal strength of frequency signal F1 is greater than a threshold value TH, while the signal strength of frequency signal F2 is still less than the threshold value TH. At this time, stylus sensing circuit 20 remains in pure hand mode. At time T2, the stylus 10 approaches or touches the touch panel. Therefore, the detection circuit 220 determines that the signal strength of both frequency signals F1 and F2 is greater than the threshold value TH. At this time, the conditions for entering stylus mode are met, so the stylus sensing circuit 20 switches to stylus mode. When the stylus sensing circuit 20 enters stylus mode, it is assumed that the detection circuit 220 selects frequency signal F1 for signal processing and calculation. Then, at time point T3, frequency signal F1 experiences a sharp signal drop due to noise interference. Therefore, the detection circuit 220 can determine that the signal strength of frequency signal F1 is less than the threshold value TH, while the signal strength of frequency signal F2 is still greater than the threshold value TH. At this time, the stylus sensing circuit 20 remains in stylus mode. Since frequency signal F1 is determined to have noise interference, the detection circuit 220 can switch to selecting frequency signal F2 for signal processing and calculation, thereby improving the stability of stylus sensing operation. At time T4, the stylus 10 leaves the touch panel. Therefore, the detection circuit 220 determines that the signal strengths at the frequencies of frequency signals F1 and F2 are both less than the threshold value TH. At this point, the condition for leaving the stylus mode is met, so the stylus sensing circuit 20 leaves the stylus mode and switches to pure hand mode. In the above embodiments, the same threshold value TH is used to determine whether to switch from pure hand mode to stylus mode and from stylus mode to pure hand mode. Those skilled in the art will understand that different threshold values ​​can also be used for these two scenarios. For example, in pure hand mode, a threshold value TH1 can be used to determine whether the stylus sensing circuit 20 should switch to stylus mode; while in stylus mode, another threshold value TH2 can be used to determine whether the stylus sensing circuit 20 should switch to pure hand mode. The threshold values ​​TH1 and TH2 can be the same or different, and can be controlled independently or correspondingly. It is worth noting that the purpose of this invention is to provide a stylus and a stylus sensing circuit. The stylus can output a downlink signal containing multiple frequency signals, and the stylus sensing circuit can process and demodulate the downlink signal to recover the frequency signals. Those skilled in the art can make modifications or variations accordingly, and are not limited thereto. For example, in the above embodiments, a downlink signal can be generated by combining two or three frequency signals. In fact, the number of frequency signals that the downlink signal can contain is not limited thereto. Furthermore, in the above embodiments, a frame period has a time slot allocated for stylus detection, wherein the detection circuit can process the downlink signal to obtain the signal strength of each frequency signal on a portion of the sensing pad, while another time slot is provided for finger touch detection. In another embodiment, a frame period can include multiple time slots allocated for stylus detection, and / or a time slot for receiving downlink signals from multiple sets of sensing pads. Based on the protocol of the stylus control system, the configuration and implementation of the time slots can be predetermined. It should be noted that the unidirectional stylus continuously outputs downlink signals, so the stylus sensing circuit can receive downlink signals at any time. In one embodiment, the detection circuit may select a frequency signal for calculation to determine the coordinates of the stylus. In another embodiment, after acquiring frequency signals of different frequencies, the detection circuit may use the different frequency signals for calculation to generate coordinates corresponding to different frequencies. In this case, the detection circuit may select the coordinates corresponding to a specific frequency that is not affected by noise as the stylus positioning result, or combine the coordinates corresponding to several frequencies that are not affected by noise to generate the stylus positioning result. It is worth noting that the above embodiments are applied to unidirectional styluses that perform one-way transmission. In another embodiment, the implementation of combining multiple frequency signals in a downlink signal can also be applied to bidirectional styluses. For example, it can use Wacom's WGP protocol, the Universal Stylus Initiative (USI) protocol, or any other protocol for bidirectional communication. The downlink signal output by the bidirectional stylus can also contain multiple frequency components, and the demodulation circuit of the stylus sensing circuit can reconstruct each frequency component to determine the presence of the stylus and detect noise interference. By including more frequencies in the downlink signal of the bidirectional stylus, noise immunity can still be improved. Figure 7 illustrates stylus detection for a unidirectional stylus and a bidirectional stylus. As shown in Figure 7, a unidirectional stylus can continuously output a downlink signal V_DL1 without receiving any uplink signal, where the downlink signal V_DL1 can be a combination of multiple frequency signals. In contrast, a bidirectional stylus can receive an uplink signal V_UL from the stylus sensing circuit and output a downlink signal V_DL2 to the stylus sensing circuit. Therefore, the stylus sensing circuit can be assigned an uplink period, a downlink period, and a finger touch (or hand touch) detection period. For a bidirectional stylus, it can receive an uplink signal V_UL from the stylus sensing circuit. The uplink signal V_UL indicates the time interval used to subsequently receive the downlink signal V_DL2 (i.e., the downlink period allocated to the stylus sensing circuit). Therefore, the bidirectional stylus can output the downlink signal V_DL2 within the specified time interval. The downlink signal V_DL2 output by the bidirectional stylus can be a combination of multiple single-frequency signals with different frequencies. Alternatively, the downlink signal V_DL2 can be a combination of multiple data with different frequencies, such as coded data embedded with carriers at different frequencies. The data can contain arbitrary information, such as the stylus's position and the pressure measured by the stylus. Regardless of whether the downlink signal V_DL2 consists of single-frequency signals or coded data, a multi-frequency implementation can achieve better noise reduction. In one embodiment, a bidirectional stylus employing a multi-frequency implementation can be a simplified device, wherein the uplink signal transmitted by the stylus has only a simple format to carry simplified information, or only contains an uncoded single-frequency signal. This type of stylus may not support frequency hopping or other complex functions, but it can support timing synchronization, which allows the stylus to output downlink signals within a time interval specified by the stylus sensing circuit, achieving noise reduction without the need for frequency hopping. In summary, this invention proposes a noise-resistant stylus sensing circuit and a stylus. The stylus can output a downlink signal containing one of multiple frequency signals of different frequencies. The stylus sensing circuit can receive the downlink signal and extract the frequency signal from the downlink signal through demodulation. Based on the noise detection results, the detection circuit of the stylus sensing circuit can select the optimal frequency signal determined to be free from noise interference for signal processing and related calculations for positioning. When the stylus sensing circuit operates in pure hand mode, the detection circuit can detect the frequency corresponding to each frequency signal to determine whether a stylus is near or in contact with the touch panel, and simultaneously determine whether any noise is present. Only when every frequency signal contained in the downlink signal is detected is it determined that a stylus has been detected. If a signal is detected only at some frequencies, this signal is determined to be noise. When the stylus sensing circuit operates in stylus mode, the stylus sensing circuit can select an optimal frequency signal for signal processing based on the signal variation of each frequency signal. The implementation of multi-frequency downlink signals can be applied to unidirectional styluses that only support downlink transmission or bidirectional styluses that simultaneously support uplink and downlink transmission. The above description is merely a preferred embodiment of the present invention; all equivalent variations and modifications made within the scope of the claims of this invention should be considered within the scope of this invention. 10: Stylus; 102_1~102_n: Signal Generator; 104: Mixer; 106: Digital-to-Analog Converter; F1~Fn: Frequency Signal; V_DL, V_DL1, V_DL2: Downlink Signal; 20: Stylus Sensing Circuit; 200: Receiver Circuit; 210: Demodulation Circuit; 220: Detection Circuit; 202: Analog Front-End Circuit; 204: Sample and Hold Circuit; V_N: Noise; PI1~PI4: Pressure Information; TH: Threshold Value; FP1~FP6: Frame Period; VAR: Signal Change; T1~T4: Time Point; V_UL: Uplink Signal Figure 1 is a schematic diagram of a stylus according to an embodiment of the present invention. Figure 2 is a schematic diagram of a stylus sensing circuit according to an embodiment of the present invention. Figure 3 illustrates the stylus transmitting pressure information to the stylus sensing circuit via a downlink frequency signal. Figure 4A illustrates stylus detection performed by the stylus sensing circuit according to an embodiment of the present invention. Figure 4B illustrates noise detection performed by the stylus sensing circuit according to an embodiment of the present invention. Figure 5 illustrates frequency selection in stylus mode according to an embodiment of the present invention. Figure 6 illustrates the integration of stylus detection and noise detection based on downlink signals. Figure 7 illustrates stylus detection for a simple stylus and a bidirectional stylus. 20: Stylus sensing circuit 200: Receiver circuit 210: Demodulation circuit 220: Detection circuit 202: Analog Front-End Circuit 204: Sample and Hold Circuit V_DL: Downlink signal F1, F2: Frequency signals V_N: Noise

Claims

1. A stylus sensing circuit, comprising: a receiving circuit for receiving a downlink signal from a stylus, the downlink signal including a plurality of frequency signals; a demodulation circuit coupled to the receiving circuit for demodulating the downlink signal to extract each frequency signal from the plurality of frequency signals; and a detection circuit coupled to the demodulation circuit for receiving the plurality of frequency signals from the demodulation circuit, selecting a first frequency signal from the plurality of frequency signals for signal processing, and ignoring a second frequency signal from the plurality of frequency signals that is determined to be subject to noise interference.

2. The stylus sensing circuit as described in claim 1, wherein each of the plurality of frequency signals is a single-tone signal.

3. The stylus sensing circuit as claimed in claim 1, wherein the detection circuit detects a noise corresponding to each of the plurality of frequency signals, and selects the first frequency signal for signal processing based on the detection result of one of the noise signals.

4. The stylus sensing circuit as described in claim 3, wherein the detection circuit selects the first frequency signal that is not interfered with by the noise for signal processing.

5. The stylus sensing circuit as described in claim 1, wherein the stylus sensing circuit is in a pure hand mode, and when the detection circuit detects that the signal strength at the frequency of each of the plurality of frequency signals is greater than a threshold value, it decides to switch to a stylus mode.

6. The stylus sensing circuit as described in claim 5, wherein when the detection circuit detects that the signal strength at the frequency of at least one of the plurality of frequency signals is less than the threshold value, the stylus sensing circuit remains in the pure hand mode.

7. The stylus sensing circuit as described in claim 5, wherein the plurality of frequency signals includes a third frequency signal and a fourth frequency signal, and when the detection circuit detects that the signal strength at one of the third frequency signals at a third frequency is greater than the threshold and the signal strength at one of the fourth frequency signals at a fourth frequency is less than the threshold, it determines that there is noise at the third frequency of the third frequency signal.

8. The stylus sensing circuit as described in claim 7, wherein the detection circuit selects the fourth frequency signal for signal processing.

9. The stylus sensing circuit as described in claim 1, wherein the stylus sensing circuit is in a stylus mode, and when the detection circuit detects that the signal strength at the frequency of each of the plurality of frequency signals is less than a threshold value, it decides to switch to a pure hand mode.

10. The stylus sensing circuit as claimed in claim 9, wherein when the detection circuit detects that the signal strength at the frequency of at least one of the plurality of frequency signals is greater than the threshold value, the stylus sensing circuit remains in the stylus mode.

11. The stylus sensing circuit as described in claim 9, wherein the plurality of frequency signals includes a third frequency signal and a fourth frequency signal, and when the detection circuit detects that the signal strength at one of the third frequency signals at a third frequency is less than the threshold and the signal strength at one of the fourth frequency signals at a fourth frequency is greater than the threshold, it determines that there is noise at the third frequency of the third frequency signal.

12. The stylus sensing circuit as described in claim 11, wherein the detection circuit selects the fourth frequency signal for signal processing.

13. The stylus sensing circuit as claimed in claim 1, wherein the detection circuit further detects a signal change in one of the plurality of frequency signals to select the first frequency signal for signal processing based on the signal change.

14. The stylus sensing circuit as described in claim 13, wherein the detection circuit selects a frequency signal with the smallest signal variation among the plurality of frequency signals for signal processing.

15. The stylus sensing circuit as claimed in claim 1, wherein the receiving circuit includes: an analog front-end (AFE) circuit for sampling the downlink signal; and an analog-to-digital converter (ADC) coupled to the analog front-end circuit for converting the downlink signal into a digital form and outputting the downlink signal to the demodulation circuit.

16. The stylus sensing circuit as described in claim 1, wherein the detection circuit performs the signal processing to obtain a coordinate of the stylus on a touch panel.

17. The stylus sensing circuit as claimed in claim 1, wherein the detection circuit selects a single frequency signal from the plurality of frequency signals for signal processing.

18. The stylus sensing circuit as described in claim 1, wherein the first frequency signal and the second frequency signal carry the same information.

19. The stylus sensing circuit as described in claim 1, wherein the first frequency signal and the second frequency signal carry different information.

20. A stylus comprising: a plurality of signal generators for generating a plurality of frequency signals having different frequencies; and a mixer coupled to the plurality of signal generators for combining the plurality of frequency signals to generate a downlink signal; wherein, A first frequency signal and a second frequency signal among the plurality of frequency signals carry the same information and are transmitted to a stylus sensing circuit. The stylus sensing circuit includes: a demodulation circuit for demodulating the downlink signal to extract each frequency signal among the plurality of frequency signals; and a detection circuit for receiving the plurality of frequency signals from the demodulation circuit, selecting the first frequency signal for signal processing, and ignoring the second frequency signal that is determined to be subject to noise interference.

21. The stylus as described in claim 20, wherein each of the plurality of frequency signals is a single-tone signal.

22. The stylus as described in claim 20 further includes: a digital-to-analog converter (DAC) coupled to the mixer for converting the downlink signal into an analog form and outputting the downlink signal.

23. The stylus as described in claim 20, wherein each of the plurality of frequency signals carries the same information.

24. The stylus as described in claim 20, wherein the second frequency signal and a third frequency signal among the plurality of frequency signals carry different information.

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