serial help ?

[Sergey Babkin]

>
>I'm working on some custom hardware and I'm getting garbled console
>output.
>
>I noticed that siocntxwait looks like this:
>
>static void
>siocntxwait(iobase)
>	Port_t	iobase;
>{
>	int	timo;
>
>	/*
>	 * Wait for any pending transmission to finish.  Required to avoid
>	 * the UART lockup bug when the speed is changed, and for normal
>	 * transmits.
>	 */
>	timo = 100000;
>	while ((inb(iobase + com_lsr) & (LSR_TSRE | LSR_TXRDY))
>	       != (LSR_TSRE | LSR_TXRDY) && --timo != 0)
>		;
>}
>
>Shouldn't there be some sort of DELAY in there?
>
>My platform has an emulated serial device in hardware, so it
>may be that the loop could run a LOT faster than transmit can
>happen...
>
>any ideas of what the DELAY should be?

I would do something like delay(1) in the loop after
this one. The idea being that if the output buffer 
is empty os nearly empty, the first loop will exit
quickly and skip the second one. Otherwise it would
go into the slow loop with delay().

Then for the 2nd loop count limit I guess take the size of the 
hardware buffer, multiply by 10 (8 data bits + 2 
start/stop), add a little for safety and divide the bit rate 
by that, and then divide the length of delay(1) by that. Or however 
long it takes for your device to transmit. If the 
actual transmission happens faster, it will set the 
TXRDY bit and the loop will complete faster.

-SB