calibrate oscillator

just measure frequency of PORTB2 and fiddle with the number until it's
close enough. don't forget to hold your tongue at the right angle!

diff --git a/foo.S b/foo.S
index 566d9fc..84336ca 100644 (file)
--- a/foo.S
+++ b/foo.S
@@ -30,6 +30,7 @@ SPL    = 0x3D
 SPH    = 0x3E
 CCP    = 0x3C
 CLKPSR = 0x36
+OSCCAL = 0x39
 WDTCSR = 0x31
 SMCR   = 0x3A
 TCCR0A = 0x2E
@@ -235,7 +236,7 @@ g: ; g(i, t) -> t
        end_mul:
                LSR a1 ;final shift is a common operation for all
 
-       MOV     t, a1 ;;TODO: use a1 in main() directly
+       MOV     t, a1 ;;TODO: use a1 in loop: directly
        #undef a1
        #undef a2
        RET ; TODO: replace CALL/RET with IJMP?
@@ -259,6 +260,8 @@ main: ; setup routine
        LDI     x, 0xd8
        OUT     CCP, x      ; change protected ioregs
        OUT     CLKPSR, one ; clock prescaler 1/2 (4Mhz)
+       LDI     x, 0xa7     ; determined by trial-and-error (->PORTB2)
+       OUT     OSCCAL, x   ; set oscillator calibration
        OUT     WDTCSR, zero; turn off watchdog ;;TODO: incomplete - see datasheet pg48
        ; OUT   SMCR, 2     ; sleep mode 'power down' ('idle' (default) has faster response time)