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Allow osctune.h to deal with F_CPU defines that end in 'UL' or 'L'. Also add more explanatory comments. #34

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Allow osctune.h to deal with F_CPU defines that end in 'UL' or 'L'. Also add more explanatory comments. #34

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59 changes: 39 additions & 20 deletions libs-device/osctune.h
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Original file line number Diff line number Diff line change
Expand Up @@ -44,34 +44,53 @@ more precise than one step of OSCCAL. It is therefore not suitable to tune an
Thanks to Henrik Haftmann for the idea to this routine!
*/

#ifdef __ASSEMBLER__

// F_CPU isn't always usable in assembler, because with some AVR toolchains it
// ends in 'L' or 'UL' (e.g. 12000000UL)
// This clever routine creates a new macro, "F_CPU_NUMERIC" that doens't end in
// 'UL'.
// Idea from https://www.avrfreaks.net/comment/334055#comment-334055
.set F_CPU_NUMERIC,0 // init to 0
.irpc param,F_CPU // go through all 'characters' in F_CPU
.ifnc \param,U // if not a 'U'
.ifnc \param,L // and not an 'L'
.set F_CPU_NUMERIC,F_CPU_NUMERIC*10+\param // multiply by 10, then add new digit
.endif
.endif
.endr

#define TIMER0_PRESCALING 64 /* must match the configuration for TIMER0 in main */
#define TOLERATED_DEVIATION_PPT 5 /* max clock deviation before we tune in 1/10 % */
#define TOLERATED_DEVIATION_PPT 5 /* max clock deviation before we tune in 1/10 (e.g. 5 = 0.5%) */

/* derived constants: */
#define EXPECTED_TIMER0_INCREMENT ((F_CPU / (1000 * TIMER0_PRESCALING)) & 0xff)
#define TOLERATED_DEVIATION (TOLERATED_DEVIATION_PPT * F_CPU / (1000000 * TIMER0_PRESCALING))

#ifdef __ASSEMBLER__
/* Timer ticks in 1ms - the expected number of ticks between USB SOFs ('Start Of Frames') */
#define EXPECTED_TIMER0_INCREMENT ((F_CPU_NUMERIC / (1000 * TIMER0_PRESCALING)) & 0xff)

#define TOLERATED_DEVIATION (TOLERATED_DEVIATION_PPT * F_CPU_NUMERIC / (1000000 * TIMER0_PRESCALING))

macro tuneOsccal
push YH ;[0]
in YL, TCNT0 ;[2]
lds YH, lastTimer0Value ;[3]
sts lastTimer0Value, YL ;[5]
sub YL, YH ;[7] time passed since last frame
subi YL, EXPECTED_TIMER0_INCREMENT ;[8]
push YH ;[0] Save HY. We are allowed to clobber only YL.
in YL, TCNT0 ;[2] YL = TCNT
lds YH, lastTimer0Value ;[3] YH = lastTimer0Value
sts lastTimer0Value, YL ;[5] lastTimer0Value = YL (store new timer value for next time)
sub YL, YH ;[7] YL = YL - YH (YL = time passed since last frame)
subi YL, EXPECTED_TIMER0_INCREMENT ;[8] YL = YL - EXPECTED_TIMER0_INCREMENT
#if OSCCAL > 0x3f /* outside I/O addressable range */
lds YH, OSCCAL ;[6]
lds YH, OSCCAL ;[6] HY = OSCCAL (timer calibration byte)
#else
in YH, OSCCAL ;[6] assembler modle uses __SFR_OFFSET == 0
in YH, OSCCAL ;[6] assembler modle uses __SFR_OFFSET == 0
#endif
cpi YL, TOLERATED_DEVIATION + 1 ;[10]
brmi notTooHigh ;[11]
subi YH, 1 ;[12] clock rate was too high
cpi YL, TOLERATED_DEVIATION + 1 ;[10] Compare YL with TOLERATED_DEVIATION + 1
brmi notTooHigh ;[11] Branch to notTooHigh if the result is <0
subi YH, 1 ;[12] Clock rate was too high - subtract 1 from YH (the OSCCal we loaded above).
; brcs tuningOverflow ; optionally check for overflow
rjmp osctuneDone ;[13]
rjmp osctuneDone ;[13] goto osctuneDone.
notTooHigh:
cpi YL, -TOLERATED_DEVIATION ;[13]
brpl osctuneDone ;[14] not too low
inc YH ;[15] clock rate was too low
cpi YL, -TOLERATED_DEVIATION ;[13] Compare YL with -TOLERATED_DEVIATION.
brpl osctuneDone ;[14] If it compares higher, we are in spec. No need to do anything.
inc YH ;[15] clock rate was too low - add 1 to YH (the OSCCal we loaded above).
; breq tuningOverflow ; optionally check for overflow
osctuneDone:
#if OSCCAL > 0x3f /* outside I/O addressable range */
Expand All @@ -80,7 +99,7 @@ macro tuneOsccal
out OSCCAL, YH ;[12-13] store tuned value
#endif
tuningOverflow:
pop YH ;[17]
pop YH ;[17] Restore YH.
endm ;[19] max number of cycles
#endif

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