ARM Technical Support Knowledge Articles

LPC PWM FREQUENCY

Applies to: uVision Debugger

Answer


Information in this article applies to:


QUESTION

I am working on a PWM project using the Philips 87LPC768. In the uVision Debugger, the reported PWM frequency does not appear to be correct.

ANSWER

The following program:

void main( void )
{
        CNSW0 = 0xFF;                                   //PWM Clock Count set to Maximum.
        CNSW1 = 0X03;

        CPSW0 = 0xCC;                                   //Setting duty cycle of channel 0 to 80%
        CPSW1 = 0x98;                                   //Setting duty cycle of channel 1 to 60%
        CPSW2 = 0x64;                                   //Setting duty cycle of channel 2 to 40%
        CPSW3 = 0x30;                                   //Setting duty cycle of channel 3 to 20%

        CPSW4 = 0xE4;                                   //Setting 8-9 bits of output registers

        PWMCON0 |= 0x40;                                //Setting all channels to non-iverted.

        PWMCON1 &= 0x00;                                //Braking disabled  Shadow Memory transfered.

        PWMCON0 |= 0x80;                                //RUN PWM.

        while(1);

}

configures the PWM to divide the CPU Clock by 1024. If the oscillator frequency is 20MHz, the CPU Clock is 3.333333MHz. Dividing by 1024 gives is PWM Frequency of approximately 3.255KHz.

This is an error. The actual value should be 19.531kHz.

This problem will be corrected in a subsequent release.

Article last edited on: 2004-06-28 21:12:10

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