[En-Nut-Discussion] AD conversion resolution?

[Gerwin Voorsluijs]

Dear all,

I'm trying to use the 10bit AD conversion of the Ethernut and with some 
trial and error I've managed to get some data out of it. The values I 
get seem OK, but I'm a little disturbed by the fact that I don't get a 
continuously climbing value out of it when I raise the voltage slowly. 
For example, for the purpose I am using it (which I am sure has 
continuous voltages!) I get the following set of "steps":

156   158   159   176   177   184   188   192   193   195   199   206 
207   224   225   227   231   240   241   248   252   254   255   304 
312   316   318   319   327   335   348   352

I know I am not using the full range of 0-5 V (I measured with a 
"normal" voltmeter around 0.8-1.7 V so that seems OK)

Does anyone have any idea what is going on? Lemme include the little 
piece of code I used:

#include <cfg/crt.h>    /* Floating point configuration. */
#include <string.h>
#include <stdio.h>
#include <io.h>
#include <dev/board.h>
#include <sys/timer.h>
#include <dev/adc.h>

int main(void)
{
     u_long baud = 115200;
     FILE *uart;

     u_char ad_chk;
     u_short adc_value;
     NutRegisterDevice(&DEV_UART, 0, 0);
     uart = fopen(DEV_UART_NAME, "r+");
     _ioctl(_fileno(uart), UART_SETSPEED, &baud);

     ADCInit();
     ADCSetRef(AVCC);
     ADCSetMode(SINGLE_CONVERSION);
     ADCSetChannel(0);

     for (;;)
	{
	ADCStartConversion();
	while (ADCRead(&adc_value))
	    NutThreadYield();
	fprintf(uart, "%i ", (int) adc_value);
     	}
}