reading in from serial port

lackofcolour

hi there, i'm looking to read into the sx28 using a serial port at 9600 baud and store the data as a byte in my program. i'm having problems with the timing - i was trying to read in a bit and then delay for approx. 104us, but it isn't working. is there a better way of doing this? any help would be greatly appreciated. thanks.

Beau Schwabe

Perhaps a snip of your code would be helpful. Don't forget the STOP and START bits that make part of the RS232 signal, so that 10 bits actually make a byte. Look for the START bit as an edge detection, and then
offset your delay for 1/2 of your bit time, in your case this would be 52uS. This way your timing will be placed in the center of each of the remaining bits and you will be less likely to "picket fence" your signal.

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

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Bean

If you want you can use SX/B to do it, or just to see what code it generates. That's what it's for.
Bean.

lackofcolour

Thanks for the help, I'll check out the SX/B.

Beau Schwabe said...
Perhaps a snip of your code would be helpful. Don't forget the STOP and START bits that make part of the RS232 signal, so that 10 bits actually make a byte. Look for the START bit as an edge detection, and then
offset your delay for 1/2 of your bit time, in your case this would be 52uS. This way your timing will be placed in the center of each of the remaining bits and you will be less likely to "picket fence" your signal.

I don't have all of the code on this computer, but the program basically loops until the start bit edge is detected and then calls a function which reads in each bit. I'll try putting a 52uS delay before this process starts. I took the delay from the sample code on this site. It looks like this:

delay
; This function delays for ((W-1)*20us), plus/minus a few ns
mov dlycnt1,W
delay1
decsz dlycnt1
jmp loop1
retp

loop1
mov w,#166
mov dlycnt2,W

loop;
nop
nop
nop
decsz dlycnt2
jmp loop
jmp delay1

Would it make sense to replace the '166' with a '6' for an approx delay of 104uS? It seems a little bit too easy, so I'm skeptical.

Again, thanks for all your help.

James Newton

http://www.sxlist.com/techref/piclist/codegen/delay.htm

or specifically:

http://www.piclist.com/cgi-bin/delay.exe?Delay=.000104&Type=seconds&Regs=d1+d2+d3+d4&clock=50&name=Delay&CPU=SX

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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!

lackofcolour

Cool, thanks for the help.

Bean

What frequency are you running the SX at ?
Bean.

PJMonty

Lackofcolour,

You asked if there is a better way of reading serial data. Yes, don't re-invent the wheel. Just use some pre-existing UART code. Heres'a link:

www.sxlist.com/techref/scenix/lib/io/osi2/serial/rs232_sx.htm

This page is full of UARTs. Furthermore, even if you want to roll your own, take a look at what others have done to get some idea of the good practices for writing your own UART.

Thanks, PeterM