AN1711 datasheet(21/40 Pages) STMICROELECTRONICS

21 / 40 page

21/40

SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

Programming Tips

1. If you use the timer interrupt to exit from WAIT mode, take care that the time period should

not be defined in such a way that half the conversion is done in WAIT mode and rest when

the CPU is running. Otherwise you will not get the benefit of a conversion done in WAIT

mode.

2. You need to consider the execution done prior to entering WAIT mode. For example, the

ADC conversion is restarted and the WFI instruction is executed. This takes 7 cycles which

is approximately 1µs when fCPU=8 MHz.

So, the time programmed in output compare register must be equal to 1us+ADC conversion

time for one ADC conversion.

For 2 ADC conversions, the time will be 1µs+2*ADC conversion time.

3. Please note that the timer reset value is 0xFFFC. This means that it will take 4 timer cycles

to overflow and reset to 0000.

4. On some ST7 devices, the ADC has an End of Conversion interrupt capability, in this case

you can use the ADC interrupt directly to wake up from WAIT mode.

Figure 12. Illustration of Wait mode

In the software example, we take the time to do two ADC conversions while the CPU is in

WAIT mode. For ST72F324, this is around 16µs. (= 1µs+ 2*7.5µs). So, the timer compare

**Rows starting with ‘W’ are results from WAIT mode execution

**Rows starting with ‘A’ are results from execution from flash.

First column

is Maximum

value recorded

Second column

is minimum

value recorded

Average

Result

Difference

(Max - Min)

AN1711 Fiches technique(PDF) 21 Page - STMicroelectronics