From fa4b6ae0202bd7777d983553ce77fe65ce94ae6c Mon Sep 17 00:00:00 2001
From: XANTRONIX Development <dev@xantronix.com>
Date: Sat, 4 Jun 2016 23:58:07 -0500
Subject: [PATCH] Ensure clock pulses occur at ~8192Hz
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Other changes:

    * Rip out checksum routine; it's a lot better to rely on the Game Boy
      Printer returning the device identifier and status bitfield to
      resend packets when checksum error bit is set

    * Ensure incoming value is read just ahead of rising edge (but I was
      already doing that so like yeah)

    * Ensure 270µs interval after packet transmission
---
 avr/Makefile |  2 +-
 avr/send.c   | 35 ++++++++---------------------------
 2 files changed, 9 insertions(+), 28 deletions(-)

diff --git a/avr/Makefile b/avr/Makefile
index c1277a8..9daccfe 100644
--- a/avr/Makefile
+++ b/avr/Makefile
@@ -8,7 +8,7 @@ CROSS		= avr-
 
 CC		= $(CROSS)gcc
 CFLAGS		= $(CGFLAGS) -Wall -Os -mmcu=atmega328p -I$(INCLUDE_PATH) \
-		  -DF_CPU=$(CLOCK_SPEED)
+		  -DF_CPU=$(CLOCK_SPEED) -DUSE_2X
 LDFLAGS		=
 
 OBJCOPY		= $(CROSS)objcopy
diff --git a/avr/send.c b/avr/send.c
index be6442f..c0dac07 100644
--- a/avr/send.c
+++ b/avr/send.c
@@ -68,10 +68,10 @@ static uint8_t spi_send_byte(uint8_t value) {
             PORTB &= ~(1 << PORTB4);
         }
 
-        value <<= 1;
-        ret   <<= 1;
+        _delay_us(61);
 
-        _delay_us(60);
+        ret   <<= 1;
+        value <<= 1;
 
         if (PINB & (1 << PINB3)) {
             ret |= 1;
@@ -79,11 +79,9 @@ static uint8_t spi_send_byte(uint8_t value) {
 
         PORTB |= (1 << PORTB5);
 
-        _delay_us(60);
+        _delay_us(61);
     }
 
-    _delay_us(150);
-
     return ret;
 }
 
@@ -104,22 +102,7 @@ static void spi_send_packet() {
     response.device = spi_send_byte(0);
     response.status = spi_send_byte(0);
 
-    _delay_us(270);
-}
-
-static uint16_t checksum() {
-    uint16_t sum = 0;
-    size_t i;
-
-    for (i=2; i<6; i++) {
-        sum += header.data[i];
-    }
-
-    for (i=0; i<header.size; i++) {
-        sum += body[i];
-    }
-
-    return sum;
+    _delay_us(244);
 }
 
 int main() {
@@ -224,12 +207,10 @@ int main() {
                     i = 0;
                     b = 0;
 
-                    if (sum == checksum()) {
-                        spi_send_packet();
+                    spi_send_packet();
 
-                        uart_putchar(response.device, NULL);
-                        uart_putchar(response.status, NULL);
-                    }
+                    uart_putchar(response.device, NULL);
+                    uart_putchar(response.status, NULL);
                 }
             }
         }