Spread spectrum clock generating circuit
First Claim
1. A spread spectrum clock generating circuit, comprising:
- an external PLL (phase-locked loop); and
an internal PLL,wherein the external PLL comprises a phase detector, a low-pass filter, a voltage-controlled oscillator and a frequency divider, each of them connecting successively, wherein the frequency divider is connected to the phase detector in order to form an external loop;
the internal PLL comprises the phase detector, the low-pass filter and the voltage-controlled oscillator of the external PLL, each of them connecting successively, wherein an output terminal of the voltage-controlled oscillator is connected with a counter, and an output terminal of the counter is connected to an input terminal of the oscillator in order to form an internal loop,wherein a direct outputting clock of the voltage-controlled oscillator outputs a spread spectrum clock through an output buffer;
the outputted spread spectrum clock is also sent to the counter;
after counting M clock cycles, the counter outputs a pulse signal and sends the pulse signal into the input terminal of the voltage-controlled oscillator so as to form the internal loop,wherein M is a natural number selected randomly.
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Accused Products
Abstract
A spread spectrum generating circuit comprises an external PLL and an internal PLL. The external PLL comprises a phase detector, a low-pass filter, a voltage-controlled oscillator and a frequency divider, each of them connecting successively. The frequency divider is connected to the phase detector in order to form an external loop. The internal PLL comprises the phase detector, the low-pass filter and the voltage-controlled oscillator of the external PLL, each of them connecting successively. An output terminal of the voltage-controlled oscillator connects with a counter, and the output terminal of the counter connects to an input of the oscillator in order to form an internal loop. The present invention is compatible with the conventional ones, and has lower design risk and higher circuit reliability; compared with the general circuit, it has drastically reduced the area and power consumption, which allows more flexible design and meets more demands.
1 Citation
5 Claims
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1. A spread spectrum clock generating circuit, comprising:
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an external PLL (phase-locked loop); and an internal PLL, wherein the external PLL comprises a phase detector, a low-pass filter, a voltage-controlled oscillator and a frequency divider, each of them connecting successively, wherein the frequency divider is connected to the phase detector in order to form an external loop; the internal PLL comprises the phase detector, the low-pass filter and the voltage-controlled oscillator of the external PLL, each of them connecting successively, wherein an output terminal of the voltage-controlled oscillator is connected with a counter, and an output terminal of the counter is connected to an input terminal of the oscillator in order to form an internal loop, wherein a direct outputting clock of the voltage-controlled oscillator outputs a spread spectrum clock through an output buffer; the outputted spread spectrum clock is also sent to the counter; after counting M clock cycles, the counter outputs a pulse signal and sends the pulse signal into the input terminal of the voltage-controlled oscillator so as to form the internal loop, wherein M is a natural number selected randomly. - View Dependent Claims (2)
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3. A spread spectrum clock generating circuit, comprising:
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an external PLL (phase-locked loop); and an internal PLL, wherein the external PLL comprises a phase detector, a low-pass filter, a voltage-controlled oscillator and a frequency divider, each of them connecting successively, wherein the frequency divider is connected to the phase detector in order to form an external loop; the internal PLL comprises the phase detector, the low-pass filter and the voltage-controlled oscillator of the external PLL, each of them connecting successively, wherein an output terminal of the voltage-controlled oscillator is connected with a counter, and an output terminal of the counter is connected to an input terminal of the oscillator in order to form an internal loop, wherein when working normally, once the external PLL gets into a locked status, the voltage-controlled oscillator controls and keeps a constant voltage, and then outputs a spread spectrum clock into the counter; after counting M clock cycles, the counter outputs a pulse signal and simultaneously gets cleared. - View Dependent Claims (4, 5)
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Specification