AVR32: prescalers for ticks generation

It's for AVR32 with usage of ASF. If we need to generate ticks, we can use one of wave generation mode of AVR32 TCs. For this we need to define value of RC register and to select clock source (internal line). We use PBA line divided by 2, 8, 32, 128 dividers (so sources are TC_CLOCK_SOURCE2..5 in ASF 'terminology' - macroses :). Here is the source of calculator of RC, divider:

#include <stdint.h>
#include <limits.h>

// max value of RC register with type uint16_t (unsigned short of AVR32 GCC)
#define MAX_RC (1<<(sizeof(uint16_t)*CHAR_BIT))
int
calc_tc_params(long *freq, long *err, uint16_t *presc, uint16_t *rc)
{
        static const long divs[] = {2, 8, 32, 128};
        static const int ndivs = sizeof divs/sizeof divs[0];
        long _err = 0, _merr = *err;
        uint16_t _rc = 0, _mrc = 0, _idiv = 0, _midiv = 0;

        for (_idiv = 0; _idiv < ndivs; _idiv++) {
                _rc = (FPBA / divs[_idiv]) / (*freq);
                _rc = (_rc==0? 1 : _rc>MAX_RC? MAX_RC : _rc);
                _err = abs((*freq) - (FPBA / divs[_idiv]) / (long)_rc);
                if (_err <= _merr) {
                        _merr = _err;
                        _midiv = _idiv;
                        _mrc = _rc;
                }
                if (!_err) break;
        }

        if (_mrc) {
                *rc = _mrc;
                *err = _merr;
                *presc = divs[_midiv];
                *freq = (FPBA / divs[_midiv]) / _mrc;
                return (1);
        } else
                return (0);
}

NOTE: FPBA is the frequency of PBA line. Inputs are:

freq - frequency in Hz
err - absolute error in Hz

Outputs are:

freq - real frequency for calculated params
err - real absolute error
presc - divider value (2..128)
rc - value for RC register to load

Timer source can be found in this way:

#define PRESC2TCSRC(PRESC) ((PRESC)==2? TC_CLOCK_SOURCE_TC2 \
                            :(PRESC)==8? TC_CLOCK_SOURCE_TC3 \
                            :(PRESC)==32? TC_CLOCK_SOURCE_TC4 \
                            :(PRESC)==128? TC_CLOCK_SOURCE_TC5 \
                            :-1)

and it's result should be used as tcclks member of tc_waveform_opt_t struct. On success, returns 1, 0 otherwise. For more information how to start 'ticks' generation, see ASF example TC_EXAMPLE3 of TC module.