Obsolete

Product(s)

- Obsolete

Product(s)

Obsolete

Product(s)

- Obsolete

Product(s)

AN2326

Calibration software

3/20

1

Calibration software

1.1

Software principle

The software algorithm, described in the following flowchart (see Figure 3.), uses the mains

frequency as a timebase. This timebase allows the microcontroller to test if the RC oscillator

frequency is above or below the targeted frequency and repeatedly transforms it by

dichotomous analysis so that in 10 iterations the RCCR/SICSR registers are set to the

optimum value. In order to simplify the calculation, the calibration is done at 1MHz, the

8MHz internal RC is divided by 8 thanks to the AVTCHR register.

As the timer speed depends on the RC oscillator frequency, it is easy to determine if the

oscillator is too fast or too slow. The counted value can be obtained by the following

equation:

Since the frequency of the counter is the frequency of the oscillator divided by 32 (only the 8

MSbits are saved on the 13 bits of the counter), if the oscillator is at 1 MHz, the result of the

count between two edges (which have a 10 ms interval), is 138h for the European standard

(220V/50Hz). For the US standard (110V/60Hz) the right value is 104h. Since the goal of the

software is to set the RC oscillator frequency to 1 MHz it means obtaining 138h as the result

of the count. So if the result of the count is greater than 138h, it means that the frequency is

too high so the program increases the value of RCCR/SICSR registers in order to decrease

the RC oscillator frequency. And if the result is less than 138h, the RCCR/SICSR register

value is decreased in order to increase the RC oscillator frequency.

Figure 1.

Dichotomous analysis of RCCR value

The RCCR register is set to 80h initially by the program, then the dichotomization starts by

adding or subtracting 40h and after each iteration the result is divided by two, so that after

10 iterations the value of RCCR is set with an accuracy of one bit.

countedvalue

f

cpu

32

f

mains

×

----------------------------

=

80h

Start Value

FFh

0h

increase oscillator

frequency

decrease oscillator

frequency

RCCR Register