[En-Nut-Discussion] timer / clock accuracy

[Michael Müller]

Hi,

using NutOS3.9.9 with an Ethernut1.3 board and GCC I noticed that the
accuracy of the time base of the clock system is very bad. There was a
deviation of nearly 1 hour per day. Checking the sources of this part of
the OS I found out that there might be a miscalculation following makro
in:  
 arch/avr/dev/ostimer.c

/* Output compare register value. */

// former calculation
//#define OCR_VALUE       ((2 * NUT_TIMER_CRYSTAL / (NUT_TIMER_PRESCALE
                                        * NUT_TICK_NFREQ) + 1) / 2)
//hack:
#define OCR_VALUE 31

/* Calucalted number of timer ticks per second. */

// former calculation
//#define NUT_TICK_FREQ   ((2 * NUT_TIMER_CRYSTAL / OCR_VALUE /
                                        NUT_TIMER_PRESCALE + 1) / 2)

//hack:
#define NUT_TICK_FREQ    1024

It seems there is a mismatch between the former 32 prescaler and the
current OCR Value of 32. OCR seems to count from 0 to 32 resulting in 33
ticks. A quick and dirty test with 31 as ORC value and 1024 directly as
NUT_TICK_FREQ makes the clock run with a much better accuracy. I
achieved a deviation of just some seconds or less per day. 


Michael