altos: Make micropeak 'serial' interface work

I prototyped the mpserial interface on a breadboard and tuned the
circuit to register the LED correctly. Then adjusted the serial code
to send bits at the right speed and format.

The logging contents are now in hexdecimal with a CCITT CRC-16
computed to verify correct reception.

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

1 files changed, 37 insertions, 6 deletions

diff --git a/src/micropeak/ao_async.c b/src/micropeak/ao_async.c
index 04bba9e8..3556f54c 100644
--- a/src/micropeak/ao_async.c
+++ b/src/micropeak/ao_async.c
@@ -21,20 +21,51 @@
 #define AO_ASYNC_BAUD	38400l
 #define AO_ASYNC_DELAY	(uint8_t) (1000000l / AO_ASYNC_BAUD)
 
+#define LED_PORT	PORTB
+
+void
+ao_async_start(void)
+{
+	LED_PORT |= (1 << AO_LED_SERIAL);
+}
+
+void
+ao_async_stop(void)
+{
+	LED_PORT &= ~(1 << AO_LED_SERIAL);
+}
+
 void
 ao_async_byte(uint8_t byte)
 {
 	uint8_t		b;
 	uint16_t	w;
 
-	/* start bit */
-
-	/* start     data         stop */
-	w = 0x001 | (byte << 1) | 0x000;
+	/*    start           data           stop */
+	w = (0x000 << 0) | (byte << 1) | (0x001 << 9);
 
+	ao_arch_block_interrupts();
 	for (b = 0; b < 10; b++) {
-		ao_led_set((w & 1) << AO_LED_SERIAL);
+		uint8_t	v = LED_PORT & ~(1 << AO_LED_SERIAL);
+		v |= (w & 1) << AO_LED_SERIAL;
+		LED_PORT = v;
 		w >>= 1;
-		ao_delay_us(26);
+
+		/* Carefully timed to hit around 9600 baud */
+		asm volatile ("nop");
+		asm volatile ("nop");
+
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
+
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
+		asm volatile ("nop");
 	}
+	ao_arch_release_interrupts();
 }