Re: 9100 keyboard wedge (udpate 2)

On Tue, 03 Jun 2003 13:38:58 -0500, Rodger Boots <rlboots@cedar-rapids.net>
wrote:

>You may want to change your crystal frequency to some handy multiple of
>1200 baud. There's a good reason all those old com port boards used a
>1.8432 MHz crystal---it divided nicely to common baud rates.

And I strongly recommend you go with the 2313 and use the UART in an interrupt
driven mode. This would allow you to write a software synchronous routines for
the PC keyboard and run the UART in the background without the loss of
characters. Asynchronous routines (UARTs) are much more complicate to write
that the Synchronous routine needed by the PC, having written both. (Though not
for the Atmel series since 99% of their parts have UARTs, I've always chosen a
part that has one if needed.)

To receive a character in a software UART you must oversample the input bit by
at *least* four times to somewhat reliably read a character (hardware UARTs
oversample a minimum of 16 times), this pretty much means a very tight loop
during which you will miss keys being pressed on the PC's keyboard. UART
transmit routines are a bit simpler, though much stricter timing is required
than a synchronous port.

On the other hand synchronous routines (where you generate a clock and data) are
not timing critical, so if a character is received by a hardware UART, causing
an interrupt to occur in the middle of your software synchronous routine, the
extra delay won't hurt anything, and no loss of characters will result.

If you're worried about what to do with the extra I/O lines of the 2313, connect
LEDs to them and blink them as status lights as characters pass through the
board. *Everybody* likes blinking status lights. ;-)

-Zonn
Received on Tue Jun 03 17:38:11 2003