- fixed assembler code

1 files changed, 44 insertions, 21 deletions

diff --git a/libs-device/osctune.h b/libs-device/osctune.h
index 3248152..a48f884 100644
--- a/libs-device/osctune.h
+++ b/libs-device/osctune.h
@@ -14,36 +14,59 @@ how the RC oscillator can be kept in sync to the USB frame rate. The code
 shown here must be added to usbconfig.h or this header file is included from
 there. This code works only if D- is wired to the interrupt, not D+!!!
 
-You may want to store a good calibration value in EEPROM. You know that the
-calibration value is good when the first USB message is received. Do not store
-the value on every received message because the EEPROM has a limited endurance.
+This is an alternative to the osccal routine in osccal.c. It has the advantage
+that the synchronization is done continuously and that it has more compact
+code size. The disadvantages are slow synchronization (it may take a while
+until the driver works), that messages immediately after the SOF pulse may be
+lost (and need to be retried by the host) and that the interrupt is on D-
+contrary to most examples.
+
+You may want to store a good calibration value in EEPROM for the next startup.
+You know that the calibration value is good when the first USB message is
+received. Do not store the value on every received message because the EEPROM
+has a limited endurance.
+
+Notes:
+Good values for Timer 0 prescaling depend on how precise the clock must be
+tuned and how far away from the default clock rate the target clock is. For
+precise tuning, choose a low prescaler factor, for a broad range of tuning
+choose a high one. A prescaler factor of 64 is good for the entire OSCCAL
+range and allows a precision of better than +/-1%. A prescaler factor of 8
+allows tuning to slightly more than +/-6% of the default frequency and is
+more precise than one step of OSCCAL. It is therefore not suitable to tune
+an 8 MHz oscillator to 12.5 MHz.
 */
 
-#define TIMER0_PRESCALING           8 /* must match the configuration for TIMER0 in main */
-#define TOLERATED_DEVIATION_PPT     5 /* max clock deviation before we tune in 1/10 % */
+#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 % */
 /* derived constants: */
-#define EXPECTED_TIMER0_INCREMENT   ((F_CPU / TIMER0_PRESCALING) & 0xff)
+#define EXPECTED_TIMER0_INCREMENT   ((F_CPU / (1000 * TIMER0_PRESCALING)) & 0xff)
 #define TOLERATED_DEVIATION         (TOLERATED_DEVIATION_PPT * F_CPU / (1000000 * TIMER0_PRESCALING))
 
 #ifdef __ASSEMBLER__
 macro tuneOsccal
-    in      YL, TCNT0
-    lds     YH, lastTimer0Value
-    sts     lastTimer0Value, YL
-    sub     YL, YH      ; time passed since last frame
-    subi    YL, EXPECTED_TIMER0_INCREMENT
-    in      YH, OSCCAL
-    cpi     YL, (TOLERATED_DEVIATION) + 1
-    brlt    notTooHigh
-    dec     YH          ; clock rate was too high
-    rjmp    osctuneDone
+    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]
+    in      YH, OSCCAL                      ;[9]
+    cpi     YL, TOLERATED_DEVIATION + 1     ;[10]
+    brmi    notTooHigh                      ;[11]
+    subi    YH, 1                           ;[12] clock rate was too high
+;   brcs    tuningOverflow                  ; optionally check for overflow
+    rjmp    osctuneDone                     ;[13]
 notTooHigh:
-    cpi     YL, -TOLERATED_DEVIATION
-    brge    osctuneDone ; not too low
-    inc     YH          ; clock rate was too low
+    cpi     YL, -TOLERATED_DEVIATION        ;[13]
+    brpl    osctuneDone                     ;[14] not too low
+    inc     YH                              ;[15] clock rate was too low
+;   breq    tuningOverflow                  ; optionally check for overflow
 osctuneDone:
-    out     OSCCAL, YH  ; store tuned value
-    endm
+    out     OSCCAL, YH                      ;[15-16] store tuned value
+tuningOverflow:
+    pop     YH                              ;[17]
+    endm                                    ;[19] max number of cycles
 #endif
 
 #define USB_SOF_HOOK        tuneOsccal