With the SX running at 50 to 75 MIPS, I believe it would be possible to get
basic uart functions at those speeds.

The ISR calculator at
http://www.sxlist.com/techref/scenix/isrcalc.asp?clock=75&baud=500000&fErr=0
.25
Shows that with an ISR running 500kHz you still have 150 instructions
available between interrupts. You should be able to shift out a bit, count
every other interrupt and shift two other bits every other int as well as
check status, start, stops and so on, all within 150 instructions.

Are you receiving at 250 and xmitting at 500? Or the other way round?

You can transmit from multiple pins with no extra cycles by using parallel
operations. E.g. 8 tx ports take only a few more cycles than 1. Receiving is
a little more difficult, but parallel operations can still apply to some
degree.

Just to get an idea of what can be done, this little snippet is the heart of
a program that xmits at 500Kbps with a 4Mhz clock (less than a 10th the
standard speed of an SX and almost a 20th the max rated speed)
http://www.sxlist.com/lib/io/osi2/serial/rs232at500kbps_sx


; serial tx
; assumes PORTB1 initially on high state

	mov	W, >>data
	xor	data, W

	mov	W, #1
	xor	RB, W	;start bit

	snb	C
	xor	RB, W	;b0

	snb	data.0
	xor	RB, W	;b1

	snb	data.1
	xor	RB, W	;b2

	snb	data.2
	xor	RB, W	;b3

        .
        .
        .

	nop
	setb	RB.0	;


And it won't kill you to add the SX to your bag of tricks, see my other
post... They just dropped the price of the dev kit to $99 for the deluxe or
$79 for the debugger (like ISD) only.

---
James Newton, Host of SXList.com
james@sxlist.com 1-619-652-0593 fax:1-208-279-8767 
SX FAQ / Code / Tutorials / Documentation: 
http://www.sxlist.com Pick faster!


 
 

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Harold Hallikainen
> Sent: 2005 Jan 21, Fri 09:15
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Favorite dual UART?
> 
> Two of the uarts need to run at 250kbps and one at 500kbps. 
> Can an SX really do that in software?
> 
> Thanks!
> 
> Harold
> 
> > Or just go with the SX and do up to 8 UARTs in software.  Certain 
> > restrictions apply, of course, but doing dual UARTs in software on a
> > SX-18 that can be had for under two bucks is certainly worthy of 
> > consideration...
> >
> 
> 
> --
> FCC Rules Updated Daily at http://www.hallikainen.com
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> http://mailman.mit.edu/mailman/listinfo/piclist
> 


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