DAC module issue (89LPC935)

 DAC module issue (89LPC935)

I wrote a simple code for output both channels to an osciloscope. But DAC module 0 P2.0 waveform(sawtooth) is much distorted. The other DAC module 1 (P0.4) outputs a perfect sawtooth. Any ideas (?) By the way i am sure it is not the code or the Tektronix oldwolf oscilloscope :(

Take a look at the code :

#include <REG935.H>

void main()
{
        unsigned char data x;
        //configure Port 2 as quasi-bidirectional
        P2M1 = 0x00;
        P2M2 = 0x00;

        //Configure the DAC
        ADCON0 = 0x04;          //AD0 enabled
        ADCON1 = 0x04;          //AD1 enabled
        ADMODB = 0x0C;          //DAC mode for ADC0 & 1

        //Sawtooth wave generation
        while(1)
        {
                for(x=0; x<256; x++)
                {
                        AD0DAT3 = x;    //write to conversion register
                        AD1DAT3 = x;    //write to conversion register
                }
        }
}

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 RE: DAC module issue (89LPC935)

A digital output pin can't generate a sawtooth without a low-pass filter.

How have you connected the two pins? Do you have any pull-up resistor on P2.0 that you say you run in quasi-bidirectional mode?

 RE: DAC module issue (89LPC935)

All pins are in Bidirectional Mode (Both channels).
No LPF at any channel.
At DAC0 module > Perfect sawtooth
At DAC1 module > Distorted sawtooth

The MCB900 evaluation board used, oscillator probes directly at I/O header pins.

Is it possible that Zinput of the latch chip connected to MCB900 board to distort the output(?) -- pin 1 of uC goes to pin17 of latch to drive the leds --