From: Keith Packard <keithp@keithp.com>
Date: Mon, 7 Apr 2014 02:48:07 +0000 (-0700)
Subject: altos: Replace C code attiny async output with inline asm
X-Git-Tag: 1.3.2.2~143
X-Git-Url: https://git.gag.com/?p=fw%2Faltos;a=commitdiff_plain;h=2427eae5f3b429d302fbe14f708dcbc68c851954;ds=sidebyside

altos: Replace C code attiny async output with inline asm

Using inline asm instead of C ensures that compiler changes will not
affect the timing of the serial data.

Signed-off-by: Keith Packard <keithp@keithp.com>
---

diff --git a/src/attiny/ao_async.c b/src/attiny/ao_async.c
index 3556f54c..9f7fd6d7 100644
--- a/src/attiny/ao_async.c
+++ b/src/attiny/ao_async.c
@@ -40,11 +40,75 @@ ao_async_byte(uint8_t byte)
 {
 	uint8_t		b;
 	uint16_t	w;
+	uint8_t		v;
+	uint8_t		bit;
+	uint8_t		w_hi, w_lo;
 
 	/*    start           data           stop */
 	w = (0x000 << 0) | (byte << 1) | (0x001 << 9);
 
+	w_hi = w >> 8;
+	w_lo = w;
+
 	ao_arch_block_interrupts();
+
+#if AO_LED_SERIAL != 4
+#error "expect AO_LED_SERIAL to be 4"
+#endif
+
+	/* Ok, this is a bit painful.
+	 * We need this loop to be precisely timed, which
+	 * means knowing exactly how many instructions will
+	 * be executed for each bit. It's easy to do that by
+	 * compiling the C code and looking at the output,
+	 * but we need this code to work even if the compiler
+	 * changes. So, just hand-code the whole thing
+	 */
+
+	asm volatile (
+		"	ldi	%[b], 10\n"		// loop value
+		"loop:\n"
+		"	in	%[v], %[port]\n"	// read current value
+		"	andi	%[v], %[led_mask]\n"	// mask to clear LED bit
+		"	mov	%[bit], %[w_lo]\n"	// get current data byte
+		"	andi	%[bit], 0x01\n"		// get current data bit
+		"	swap	%[bit]\n"		// rotate by 4 (AO_LED_SERIAL is 4)
+		"	andi	%[bit], 0xf0\n"		// mask off other 4 bits
+		"	or	%[v], %[bit]\n"		// add to register
+		"	out	%[port], %[v]\n"	// write current value
+		"	lsr	%[w_hi]\n"		// shift data
+		"	ror	%[w_lo]\n"		//  ...
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+
+		"	nop\n"
+		"	nop\n"
+		"	nop\n"
+		"	subi	%[b], 1\n"		// decrement bit count
+		"	brne	loop\n"			// jump back to top
+		: [v]        "=&r" (v),
+		  [bit]	     "=&r" (bit),
+		  [b]	     "=&r" (b),
+		  [w_lo]     "+r" (w_lo),
+		  [w_hi]     "+r" (w_hi)
+		: [port]     "I"  (_SFR_IO_ADDR(LED_PORT)),
+		  [led_mask] "M"  ((~(1 << AO_LED_SERIAL)) & 0xff)
+		);
+
+#if 0
+	/*
+	 * Here's the equivalent C code to document
+	 * what the above assembly code does
+	 */
 	for (b = 0; b < 10; b++) {
 		uint8_t	v = LED_PORT & ~(1 << AO_LED_SERIAL);
 		v |= (w & 1) << AO_LED_SERIAL;
@@ -54,6 +118,7 @@ ao_async_byte(uint8_t byte)
 		/* Carefully timed to hit around 9600 baud */
 		asm volatile ("nop");
 		asm volatile ("nop");
+		asm volatile ("nop");
 
 		asm volatile ("nop");
 		asm volatile ("nop");
@@ -67,5 +132,6 @@ ao_async_byte(uint8_t byte)
 		asm volatile ("nop");
 		asm volatile ("nop");
 	}
+#endif
 	ao_arch_release_interrupts();
 }