Fix pulse timing and initial state of error flag

firmware/fast-gpio.h

@@ -96,13 +96,20 @@
 #endif
 
 
+// Without the nop, these pulses happen in about 115ns.  The nop adds about 8ns
+// to the process, but without it, voltage bottoms out too high (around 1.5V).
+// To be a reliable zero on our flip-flops, we need 0.9V max, and we need the
+// low pulse width to be at least 7ns.  With this nop, the signal stays under
+// 0.9V for about 20ns.
 #define FAST_PULSE_ACTIVE_LOW(PIN) { \
   FAST_CLEAR(PIN); \
+  asm("nop"); \
   FAST_SET(PIN); \
 }
 
 #define FAST_PULSE_ACTIVE_HIGH(PIN) { \
   FAST_SET(PIN); \
+  asm("nop"); \
   FAST_CLEAR(PIN); \
 }
 

firmware/firmware.ino

@@ -292,7 +292,9 @@ void setup() {
   init_all_hardware();
 
 #ifdef RUN_TESTS
-  if (!is_error_flagged()) {
+  if (is_error_flagged()) {
+    Serial.println("Error flagged, skipping tests!\n");
+  } else {
     run_tests();
   }
 #endif

firmware/registers.cc

@@ -68,7 +68,12 @@ int is_error_flagged() {
 uint8_t read_register(int register_address) {
   FAST_WRITE(REG_PIN__A0, register_address & 1);
   FAST_WRITE(REG_PIN__A1, register_address & 2);
-
-  // FIXME: need to delay here?
+  // The data sheet for the 74HCT670 says the data will be available after at
+  // most 295ns at 2V, and 59ns at 4.5V.  (Not given for 3.3V)  Here we wait 1
+  // us to be sure that we are reading the right data.  It is okay to read a
+  // register a little more slowly.  Writing to SRAM and reading from the
+  // network are the critical operations in terms of speed.
+  // https://www.ti.com/lit/ds/symlink/cd74hct670.pdf
+  delayMicroseconds(1);
   return FAST_READ_MULTIPLE(REG_PIN__D_MASK, REG_PIN__D_SHIFT);
 }

software/player/src/segavideo_menu.c

@@ -273,6 +273,9 @@ static void loadMenuColors() {
 void segavideo_menu_init() {
   kprintf("segavideo_menu_init\n");
 
+  // Clear the error flag, which may boot up in a random state.
+  clearPendingError();
+
   // Load menu palettes.
   loadMenuColors();