USB clock generation circuit

Abstract

The invention discloses a USB clock generation circuit, which comprises a crystal oscillator circuit module and an RC oscillator module for providing a clock signal for the USB module, and is characterized in that the RC oscillator module is connected to the crystal oscillator circuit module and receives a reference clock provided by the crystal oscillator circuit module; the RC oscillator modulecomprises an RCO sub-circuit, a synchronous pulse generation sub-circuit, a counter sub-circuit, a switch control sub-circuit and a clock synthesis sub-circuit; the RCO sub-circuit outputs a first control signal; the synchronous pulse generation sub-circuit is connected to the crystal oscillation circuit module and outputs a second control signal; the counter sub-circuit is connected with the RCOsub-circuit and the synchronous pulse generation sub-circuit and outputs a third control signal; and the clock synthesis sub-circuit outputs a clock signal required by the USB module after operation.According to the technical scheme, the reference clock is provided through the crystal oscillator circuit module, and the synchronous pulse generation sub-circuit is added to enable the reference clock to synchronize the RCO sub-circuit, and the application requirement of the full-speed USB clock is met.

Description

technical field [0001] The invention relates to the technical field of integrated circuits, in particular to a USB clock generating circuit. Background technique [0002] At present, in SOC (System-on-a-Chip, system on chip), there are usually two types of methods for providing clocks for digital circuits: [0003] 1. Use a crystal oscillator to generate a reference clock, and then multiply the frequency by the internal PLL (Phase Locked Loop) to generate the required clock frequency, such as figure 1 shown. The advantage of this method is: high frequency accuracy; the disadvantage is: the start-up of the crystal oscillator is slow, usually more than 1ms, resulting in a long wait for the start-up of the digital circuit. [0004] 2. The RC oscillator is used, and after calibration, a more accurate clock is provided to the digital circuit, such as figure 2 shown. The method has the advantages of fast start-up, simple circuit and low cost. The disadvantage is: the frequen...

AI Technical Summary

Problems solved by technology

The advantage of this method is: high frequency accuracy; the disadvantage is: the start-up of the crystal oscillator is slow, usually more than 1ms, resulting in a long time to wait for the start-up of the digital circuit

The disadvantage is: poor frequency accuracy

Or limit the frequency of the crystal oscillator, 24M or 48M, save an additional phase-locked loop, and use a simple double frequency circuit to generate a 48 MHz clock, but this limits the flexibility of the product (and the higher the crystal frequency, the more expensive it is)

Method 2 does not have the above disadvantages, but it needs to achieve frequency accuracy within plus or minus 2500ppm under process / temperature deviation, which requires additional circuit design efforts and circuit area

There is also method 3, which is to add additional processing in the digital domain to reduce the requirements for the accuracy of the USB clock, but this will also increase the cost and design difficulty

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