From 765dc3ab3d79bdca696a0d651385af3020c895c6 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Sun, 28 May 2023 18:54:03 +0200
Subject: Add initial link cable clock sync

---
 Makefile        |  5 +++++
 src/gba/gba.h   | 18 ++++++++++++++++++
 src/sequencer.c | 29 +++++++++++++++++++++++++++++
 3 files changed, 52 insertions(+)

diff --git a/Makefile b/Makefile
index 90b0a0d..5ccc725 100644
--- a/Makefile
+++ b/Makefile
@@ -81,6 +81,11 @@ $(BUILD_DIR):
 run: main
 	mgba-qt $(BIN)
 
+deploy:
+	mount /dev/sdf1 /mnt
+	cp $(BIN) /mnt
+	umount /mnt
+
 # Remove build directory.
 clean:
 	rm -rf $(BUILD_DIR)
diff --git a/src/gba/gba.h b/src/gba/gba.h
index be7a305..f726b72 100644
--- a/src/gba/gba.h
+++ b/src/gba/gba.h
@@ -451,6 +451,24 @@ void irq_init(void);
 int bios_vblank_wait();
 int bios_div(int num, int denom);
 
+//
+// SIO Link Cable
+//
+
+#define SIO_MODE *((vu16*)(MEM_IO + 0x0134))
+#define SIO_CNT  *((vu16*)(MEM_IO + 0x0128))
+
+#define SIO_MODE_GP     (2 << 14)
+#define SIO_SC(X)       ((X) << 0)
+#define SIO_SD(X)       ((X) << 1)
+#define SIO_SI(X)       ((X) << 2)
+#define SIO_SO(X)       ((X) << 3)
+#define SIO_SC_OUT(X)   ((X) << 4)
+#define SIO_SD_OUT(X)   ((X) << 5)
+#define SIO_SI_OUT(X)   ((X) << 6)
+#define SIO_SO_OUT(X)   ((X) << 7)
+#define SIO_IRQ_ENABLE  (1 << 8)
+
 //
 // Sound.
 //
diff --git a/src/sequencer.c b/src/sequencer.c
index 867e686..7c989f2 100644
--- a/src/sequencer.c
+++ b/src/sequencer.c
@@ -19,6 +19,19 @@ bool redraw_bpm = true;
 bool redraw_piano_note = true;
 bool update_bpm = false;
 
+void
+gate_off(void) {
+    SIO_MODE = SIO_MODE_GP
+        | SIO_SC_OUT(1)
+        | SIO_SD_OUT(1)
+        | SIO_SI_OUT(0)
+        | SIO_SO_OUT(1)
+        | SIO_SC(0)
+        | SIO_SD(1)
+        | SIO_SO(0);
+    TIMER_CTRL_1 = 0;
+}
+
 void
 play_step(void) {
     Pattern *pat = &patterns[current_pattern];
@@ -135,6 +148,22 @@ play_step(void) {
         SOUND_NOISE_CTRL = 0;
         SOUND_NOISE_FREQ = 0;
     }
+    // TODO: This should be configurable e.g. po needs a 1 each 2 to sync
+    // exactly, but we can also just want a pseudo clock divider.
+    if (step_counter % 2 == 0) {
+        SIO_MODE = SIO_MODE_GP
+            | SIO_SC_OUT(1)
+            | SIO_SD_OUT(1)
+            | SIO_SI_OUT(0)
+            | SIO_SO_OUT(1)
+            | SIO_SC(1)
+            | SIO_SD(1)
+            | SIO_SO(0);
+        int n_ticks = -244181 / 600;
+        irs_set(IRQ_TIMER_1, gate_off);
+        TIMER_DATA_1 = n_ticks;
+        TIMER_CTRL_1 = TIMER_CTRL_IRQ | TIMER_CTRL_ENABLE | TIMER_CTRL_FREQ_3;
+    }
     step_counter = (step_counter + 1) % 16;
     redraw_piano_note = true;
 }
-- 
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