Bit Banging - read serial COM port

Carlos Q · Joined: 10 Dec 2009 Posts: 5

Hi all,

Hi I have an older board and the firmware was developed in ASM. I would like to develop that firmware using CCS but i have problems reading serial communication from several I/O pins using Bit Banging, how can I code that?? Anyone can help me with a little code for do that? Advices are appreciated.

I can't change the GPIO pins that I use for receive serial communication and they don't have interrupts. The board exists and is older.

I´m using PIC16F877A, 9600 bps for serial communication and 20 MHz.

I will appreciate your help,

Thanks a lot.

Regards,

Carlos Q.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The following page tells 3 ways to do a software UART. CCS uses the
3rd method in the list:
http://www.8052.com/faqs.phtml?FAQ=124758

Here is a simple example of a software UART on pins B0 and B1.
You must have a Max232-type chip connected between the xmit and rcv
pins and your PC's COM port.

Carlos Q · Joined: 10 Dec 2009 Posts: 5

Thanks PCM, you always help us.

Only that?

To implement Bit Banging in ASM I need to wait for start bit, stop bit and so on.

How can I ask the presence of one byte or how can I wait for the byte pretense?

In your example, with the while statement, generates an loop but how can I manage the synchronization?

Thanks again.

Carlos

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

getc() will wait forever until a Start bit arrives. You can also use kbhit() to just check and see if a Start bit is present. You should poll kbhit() at least 4x the serial bit rate to make sure you don't call getc() too late.
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John P · Joined: 17 Sep 2003 Posts: 331

If I'm understanding the requirement, in the list quoted by PCM programmer you would need to select option 1, where the incoming data is polled at some fast rate, at least 4x the baud rate. At 9600Bd that means 38400 samples/sec minimum, and that's pretty fast for a processor at 20MHz. I think you'd have to abandon the official CCS interrupt structure and use a global interrupt where you'd write all the code yourself, which is tedious and risky. It's easiest if the interrupt doesn't do much, so you might read the entire PortB and shove it into a buffer, increment an index and then get out. The challenge then would be to make sure that back in main() your code can read the buffer and process whatever's there at a rate faster than new data can accumulate.

A wild speculation--is this board receiving RFID data? Multiple receive-only serial ports sounds like it.