add serial example

serial.c

@@ -0,0 +1,47 @@
+#include <stdint.h>
+
+volatile uint8_t * SCFTDR2 = (volatile uint8_t *)0xffe8000c;
+volatile uint16_t * SCFSR2 = (volatile uint16_t *)0xffe80010;
+volatile uint16_t * SCFCR2 = (volatile uint16_t *)0xffe80018;
+
+#define SCFSR2__TDFE (1 << 5)
+#define SCFSR2__TEND (1 << 6)
+
+#define SCFCR2__TTRG(n) (((n) & 0b11) << 4)
+
+static inline void character(const char c)
+{
+  // wait for transmit fifo to become partially empty
+  while ((*SCFSR2 & SCFSR2__TDFE) == 0);
+
+  // unset TDFE bit
+  *SCFSR2 &= ~SCFSR2__TDFE;
+
+  *SCFTDR2 = c;
+}
+
+static inline void string(const char * s)
+{
+  while (*s != 0) {
+    character(*s++);
+  }
+}
+
+void main()
+{
+  // set the transmit trigger to `1 byte`--this changes the behavior of TDFE
+  *SCFCR2 = SCFCR2__TTRG(0b11);
+
+  string("hello\n");
+
+  // hack: "flush" the `hello` characters before the serial loader resets the
+  // serial interface
+  //
+  // Other methods to detect "transmit fifo is empty" appear to be
+  // non-functional (including polling SCFDR2 or SCFSR2)
+  //
+  // It would also be appropriate to use an SH7091 timer here, to delay a
+  // certain number of milliseconds, though that would require defining more
+  // registers.
+  string("  ");
+}