Software UART driver

Hi all,

I have tried to look an answer for this on the web, but I mainly got "no can do" (timing issue).

Kernel: 2.6.3x
MCU: atmel at91 (9263)

I wish to get/write a driver to bitbang a couple of gpios to emulate a uart. the communication is with a remote PIC (PIC12F683) to which I have already wrote the software and checked with linux terminal.
the communication does not have to be fast at all (1200 would be fine). only a few bytes are exchanged.

can someone point me in the right direction? did someone already develop such a driver? is there an example/guide I can follow to do it?

Daniel.

I would look up the serial driver source code for some examples. Issue being are that many chips support actual UART operation, hence all the examples may be UART register setup examples as well as BAUD rate divisor calculations. However all a serial port is, is TX, RX, and Ground. Maybe Power if the remote device requires that. You just need to understand the signaling and how to clock each bit. For that I'd determine the baud rate required and generate a clock or use a timer making sure you have less then a 1% amount of error for that, preferably as close to zero as you can reach. Then use that clock or timer to bit bang out the bit changes for your characters. The other part is getting the electrical signals all set; such as 3.3, 5, or 12 volt, understanding what signal levels you need to have mark versus space, as well as how to code each individual byte to make the result be something like N81. Therefore I'd make sure in advance that the electrical levels are correct, the capability to clock bit transitions is all set, and then make sure I understood what constitutes proper bit signaling for the serial communications method you wish to use.

Hi,

Originally Posted by lightydo Hi all, I have tried to look an answer for this on the web, but I mainly got "no can do" (timing issue). Kernel: 2.6.3x MCU: atmel at91 (9263) I wish to get/write a driver to bitbang a couple of gpios to emulate a uart. the communication is with a remote PIC (PIC12F683) to which I have already wrote the software and checked with linux terminal. the communication does not have to be fast at all (1200 would be fine). only a few bytes are exchanged. can someone point me in the right direction? did someone already develop such a driver? is there an example/guide I can follow to do it? Daniel.

It's been a while since I have written for serial device. Look at: http://en.wikibooks.org/wiki/Serial_...g/Serial_Linux

Serial Programming: Introduction and OSI Network Model -- RS-232 Wiring and Connections -- Typical RS232 Hardware Configuration -- 8250 UART -- DOS -- MAX232 Driver/Receiver Family -- TAPI Communications In Windows -- Linux and Unix -- Java -- Hayes-compatible Modems and AT Commands -- Universal Serial Bus (USB) -- Forming Data Packets -- Error Correction Methods -- Two Way Communication -- Packet Recovery Methods -- Serial Data Networks -- Practical Application Development -- IP Over Serial Connections

If you are wanting no handshakes and just a serial character communication then I would look at the Linux kernel uart.c which is for the 2.6 kernel but a well written module;

//-------------------------------------------------------------------
//    uart.c 
// 
//    A character-mode device-driver for the 16550 Serial UART. 
// 
//    NOTE: Written and tested with Linux kernel module 2.6.26. 
// 
//    programmer: ALLAN CRUSE 
//    written on: 26 AUG 2008 
//    revised on: 31 AUG 2008 -- for cables without DTR<-->DSR 
//    revised on: 26 OCT 2008 -- to include 'my_poll()' method 
//-------------------------------------------------------------------

Plus be sure to look at Linux Device Drivers, 3rd Edition to help you understand things.

EDIT: Another good place to look is: https://sites.google.com/site/biffer...software-uarts

Hope this helps.
Have fun & enjoy!

Thanks Onebuck, but in this case I am trying to bitbang gpios to emulate a uart not use an existing h/w...


I guess I will have to create another protocol based on some existing.

Hi,

I did include a link to a software UART solution. You cannot just use the GPIO without providing proper pullups for the signal lines.

In order to know how to write a driver to emulate a function you should see how the hardware is managed. You must provide the polling function along with GPIO management. 'uart.c' provides the means to understand how to work with the serial device.