millis (2.3% slow)

Author: nerdralph

src/ArduinoShrink.h

@@ -1,9 +1,14 @@
 // (c) Ralph Doncaster 2020
 // ArduinoShrink
 
+#if F_CPU != 16000000
+#error Only 16Mhz F_CPU supported
+#endif
+
 #include <util/delay.h>
 
 inline void delayMicroseconds(unsigned int us)
 {
     _delay_us(us);
 }
+

src/millis.S

@@ -0,0 +1,50 @@
+; (c) Ralph Doncaster 2020
+; ArduinoShrink
+
+; all used io registers are < 0x3F
+;#define __SFR_OFFSET 0
+#include <avr/io.h>
+
+;struct {
+;    uint32_t t0_ovfl;
+;    uint8_t ovfl_pad;
+;} t0_millis;
+
+; micros not supported
+.global micros
+micros:
+
+.global millis
+millis:
+    ldi ZL, lo8(t0_millis)
+    ldi ZH, hi8(t0_millis)
+    ld r22, Z
+    ldd r23, Z+1
+    ldd r24, Z+2
+    ldd r25, Z+3
+    ld r0, Z
+    cp r0, r22                          ; has millis changed?
+    brne millis
+ret
+
+.global TIMER0_OVF_vect
+TIMER0_OVF_vect:
+    push r16
+    in r16, SREG-0x20
+    push r16
+    push ZL
+    ldi ZL, lo8(t0_millis)
+    push ZH
+    ldi ZH, hi8(t0_millis)
+millis_inc:
+    ld r16, Z
+    subi r16, -1
+    st Z+, r16
+    brcc millis_inc
+    pop ZH
+    pop ZL
+    pop r16
+    out SREG-0x20, r16
+    pop r16
+reti
+

src/millis.c

@@ -4,49 +4,19 @@
 #include <Arduino.h>
 
 struct {
-    uint8_t fract_count;
-    volatile uint32_t millis;
+    volatile uint32_t t0_ovfl;
     uint8_t ovfl_pad;
 } t0_millis;
 
-ISR(TIMER0_OVF_vect, ISR_NAKED)
-{
-    asm (
-    "push r16                   \t\n"
-    "in r16, __SREG__           \t\n"
-    "push r16                   \t\n"
-    "push ZL                    \t\n"
-    "ldi ZL, lo8(t0_millis)     \t\n"
-    "ldd r22, Z+0               \t\n"
-    "reti                       \t\n"
-    );
-}
-
-__attribute((naked))
-uint32_t millis()
-{
-    asm (
-    "ldd r22, Z+0               \t\n"
-    "ldd r23, Z+1               \t\n"
-    "ldd r24, Z+2               \t\n"
-    "ldd r25, Z+3               \t\n"
-    "ldd r0, Z+0                \t\n"
-    "cp r0, r22                 \t\n"   // has millis changed?
-    "brne millis                \t\n"
-    "ret                        \t\n"
-    :: "z"(&(t0_millis.millis)) 
-    );
-}
-
 // portability macros for mega8
 #ifndef TCCR0B
 #define TCCR0B TCCR0
 #define TIMSK0 TIMSK
 #endif
-__attribute(( section(".init8"), used))
+
+__attribute(( section(".init8"), used, naked))
 void init_millis()
 {
-    TCCR0B = _BV(CS01);                 // div8 prescaler
+    TCCR0B = _BV(CS01) | _BV(CS00);     // div64 prescaler
     TIMSK0 = _BV(TOIE0);                // enable overflow interrupt
 }
-