UART

The WonderSwan's EXT port features an UART operating with the following configuration:

• 9,600 or 38,400 bps (bauds per second),
• 8N1 (8 data bits followed by 1 stop bit, no parity).

This allows for an effective maximum transfer speed of 960 or 3840 bytes per second, respectively.

The hardware also features one-byte transmit and receive buffers, which allow for a slight delay in code when handling data to/from the console.

Interrupts

The UART features two interrupts:

• UART Send Ready - constantly active while the transmit buffer is empty (Serial Status bit 2),
• UART Receive Ready - constantly active while the receive buffer contains a byte (Serial Status bit 0).

I/O ports

Serial Receive Data ($B1 read)

7  bit  0
---- ----
dddd dddd
|||| ||||
++++-++++- Receive buffer value

Serial Transmit Data ($B1 write)

7  bit  0
---- ----
dddd dddd
|||| ||||
++++-++++- Transmit buffer value

Serial Status ($B3 read)

7  bit  0
---- ----
eb.. .tor
||    |||
||    ||+- 1 if the receive buffer contains a byte
||    |+-- 1 on overrun (receive buffer overflow)
||    +--- 1 if the transmit buffer is empty (can transmit another byte)
|+-------- UART speed: 0 = 9600 bps, 1 = 38400 bps
+--------- UART enable: 0 = off, 1 = on

Serial Control ($B3 write)

7  bit  0
---- ----
ebO. ....
|||
||+------- Write 1 to reset overrun
|+-------- UART speed: 0 = 9600 bps, 1 = 38400 bps
+--------- UART enable: 0 = off, 1 = on