read ROM: improve post-byte code efficiency

3 files changed, 48 insertions, 36 deletions

diff --git a/Makefile b/Makefile
index c31d4ec..deba10b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,3 @@
-MCU ?= attiny2313
 AVRDUDE_PROGRAMMER ?= usbasp
 
 AVRCC ?= avr-gcc
@@ -7,8 +6,8 @@ AVRNM ?= avr-nm
 AVROBJCOPY ?= avr-objcopy
 AVROBJDUMP ?= avr-objdump
 
-CFLAGS += -mmcu=attiny2313 -DF_CPU=20000000UL
-#CFLAGS += -gdwarf-2 -fverbose-asm -save-temps
+CFLAGS += -mmcu=attiny2313a -DF_CPU=20000000UL
+#CFLAGS += -gdwarf-2 -fverbose-asm
 CFLAGS += -I. -std=gnu99 -Os -Wall -Wextra -pedantic
 CFLAGS += -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums
 # r28 .. r31 hold the microsecond counters
@@ -31,7 +30,7 @@ main.elf: main.c
 	avr-size -d $@
 
 program: main.hex main.eep
-	${AVRFLASH} -p ${MCU} -c ${AVRDUDE_PROGRAMMER} ${AVRFLAGS}
+	${AVRFLASH} -p attiny2313 -c ${AVRDUDE_PROGRAMMER} ${AVRFLAGS}
 
 secsize: main.elf
 	${AVROBJDUMP} -hw -j.text -j.bss -j.data main.elf
diff --git a/README.md b/README.md
index 3315321..07d85a0 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # owi-slave
 
-Implementation of an One Wire Interface slave device on an ATTiny 2313 without
+Implementation of an One Wire Interface slave device on an ATTiny 2313A without
 external clock.
 
 This code is a work in progress. The Command readout is not working realibly.
diff --git a/main.c b/main.c
index d2a3ef1..3c986ed 100644
--- a/main.c
+++ b/main.c
@@ -44,14 +44,16 @@ int main (void)
 {
 	/* watchdog reset after ~4 seconds */
 	MCUSR &= ~_BV(WDRF);
-	WDTCSR = _BV(WDCE) | _BV(WDE);
-	WDTCSR = _BV(WDE) | _BV(WDP3);
+	WDTCR = _BV(WDCE) | _BV(WDE);
+	WDTCR = _BV(WDE) | _BV(WDP3);
 
 	MCUCR = _BV(ISC10);
 	GIMSK = _BV(INT1);
 
 	ACSR |= _BV(ACD);
 
+	PRR = _BV(PRUSI) | _BV(PRUSART);
+
 	POS = 0;
 	APOS = 0;
 	DDRD = 0;
@@ -138,14 +140,14 @@ ISR(INT1_vect)
 
 				DDRD = _BV(PD3);
 
-				// 15us loop - r31 / r30 need not be preserved
+				// 15us loop - r31 / r28 need not be preserved
 				asm volatile ("ldi r31, 0");
-				asm volatile ("ldi r30, 15"); // Z = 120
+				asm volatile ("ldi r28, 15"); // Z = 15
 				asm volatile ("wdr"); // <-----
 				asm volatile ("wdr");
 				asm volatile ("wdr");
-				asm volatile ("sbiw r30, 1");
-				asm volatile ("cp r30, r1");
+				asm volatile ("sbiw r28, 1");
+				asm volatile ("cp r28, r1");
 				asm volatile ("brne .-12"); // -^
 
 				DDRD = 0;
@@ -156,27 +158,38 @@ ISR(INT1_vect)
 				POS <<= 1;
 			}
 			else {
-				APOS++;
 				POS = 1;
-				if (APOS == 1)
-					BYTE = ~ADDR7;
-				else if (APOS == 2)
-					BYTE = ~ADDR6;
-				else if (APOS == 3)
-					BYTE = ~ADDR5;
-				else if (APOS == 4)
-					BYTE = ~ADDR4;
-				else if (APOS == 5)
-					BYTE = ~ADDR3;
-				else if (APOS == 6)
-					BYTE = ~ADDR2;
-				else if (APOS == 7)
-					BYTE = ~ADDR1;
-
-				else if (APOS == 8) {
-					LASTCMD = 0;
-					POS = 0;
-				}
+				asm volatile ("in r28, 0x28"); // APOS
+				asm volatile ("inc r28"); // APOS++
+				asm volatile ("cpi r28, 1");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR7));
+				asm volatile ("cpi r28, 2");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR6));
+				asm volatile ("cpi r28, 3");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR5));
+				asm volatile ("cpi r28, 4");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR4));
+				asm volatile ("cpi r28, 5");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR3));
+				asm volatile ("cpi r28, 6");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR2));
+				asm volatile ("cpi r28, 7");
+				asm volatile ("brne .+2");
+				asm volatile ("ldi r29, %0" : : "i" (~ADDR1));
+				asm volatile ("out 0x1d, r29"); // BYTE
+
+				asm volatile ("cpi r28, 8");
+				asm volatile ("brne .+4");
+				asm volatile ("out 0x3c, r1"); // LASTCMD - OC0B
+				asm volatile ("out 0x2a, r1"); // BUF - OC1A
+
+				asm volatile ("out 0x28, r28"); // APOS
 			}
 		}
 		else if (LASTCMD == 0xf0) {
@@ -186,14 +199,14 @@ ISR(INT1_vect)
 			//if (BYTE & POS) {
 				DDRD = _BV(PD3);
 
-				// 15us loop - r31 / r30 need not be preserved
-				asm volatile ("ldi r31, 0");
-				asm volatile ("ldi r30, 15"); // Z = 120
+				// 15us loop - r29 / r28 need not be preserved
+				asm volatile ("ldi r29, 0");
+				asm volatile ("ldi r28, 15"); // Z = 15
 				asm volatile ("wdr"); // <-----
 				asm volatile ("wdr");
 				asm volatile ("wdr");
-				asm volatile ("sbiw r30, 1");
-				asm volatile ("cp r30, r1");
+				asm volatile ("sbiw r28, 1");
+				asm volatile ("cp r28, r1");
 				asm volatile ("brne .-12"); // -^
 
 				DDRD = 0;