diff --git a/dsp/synth.c b/dsp/synth.c
new file mode 100644
index 0000000..131b6f9
--- /dev/null
+++ b/dsp/synth.c
@@ -0,0 +1,107 @@
+#include "sys.h"
+
+void init() {
+  square0->output_address = pdmout0;
+  square0->high_val = 0xafff;
+  square0->low_val = 0x0000;
+  square0->flags = SQUARE_FLAG_ENABLE_msk;
+  // will set the period later
+}
+
+int strlen(const char* str) {
+  int i = 0;
+  while(str[i] != '\0') {
+    i++;
+  }
+  return i;
+}
+
+void log(const char* stuff) {
+  uart_writen(uart0, stuff, strlen(stuff));
+}
+
+void dump(uint8_t a) {
+  uint8_t hi = a >> 4;
+  uint8_t lo = a & 0xf;
+
+  const char* letters = "0123456789abcdef";
+
+  uart_write(uart0, letters[hi]);
+  uart_write(uart0, letters[lo]);
+}
+
+#define midi_uart uart1
+
+#define MODE_IDLE 0
+#define MODE_DOWN 1
+#define MODE_UP 2
+
+int main() {
+  const int periods[] = {
+    1493,
+    0,
+    1330,
+    0,
+    1185,
+    1119,
+    0,
+    996,
+    0,
+    887,
+    0,
+    791,
+  };
+
+  init();
+
+  led0->output = 42;
+
+  int keydown = 0;
+  int mode = MODE_IDLE;
+
+  while(1) {
+    uint8_t c = uart_read(midi_uart);
+
+    if (c == 0xfe) {
+      // nothing
+    } else if (c == 0x90) {
+      mode = MODE_DOWN;
+    } else if (c == 0x80) {
+      mode = MODE_UP;
+    } else if ((c & 0x80) == 0) {
+      if (mode == MODE_DOWN) {
+        int period = 0;
+
+        keydown = c;
+
+        log("dn ");
+        dump(c);
+        log("\r\n");
+
+        if (c > 0x3b) {
+          if (c < 0x48) {
+            period = periods[c - 0x3c];
+          }
+        }
+
+        uart_read(midi_uart);
+
+        square0->period = period;
+      } else if (mode == MODE_UP) {
+        log("up ");
+        dump(c);
+        log("\r\n");
+
+        uart_read(midi_uart);
+
+        if (keydown == c) {
+          square0->period = 0;
+        }
+      }
+    } else {
+      log("weird: ");
+      dump(c);
+      log("\r\n");
+    }
+  }
+}