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I want to pass binary data from my PC to my micro-controller through the real serial port in my computer. At this time, I'm thinking of using INB and OUTB functions so I don't have to put up with linux tty character overrides.

Question is, how do I configure the baud rate of the serial port, if I use its address with the INB and OUTB functions? For example, if I want to write to COM1 (0x3F8), how will I be able to determine the baud rate data is being written at?

Does stty still have some sort of background control with INB and OUTB or setserial or what?

old_timer
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Mike -- No longer here
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    "*At this time, I'm thinking of using INB and OUTB functions so I don't have to put up with linux tty character overrides.*" You're making things *much* more complicated for no good reason. Putting a port in raw mode is trivial. – David Schwartz Nov 01 '17 at 05:31
  • if you were doing this bare metal, on DOS, etc. then you simply read the documentation. But you tagged linux so you simply use the api calls to deal with the uart. – old_timer Nov 01 '17 at 10:54

2 Answers2

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At this time, I'm thinking of using INB and OUTB functions so I don't have to put up with linux tty character overrides.

This is a terrible, no good, very bad idea, and it won't work. The existing serial drivers will get very confused if you try to change the port configuration from under them.

Use standard UNIX APIs to interact with the serial port. It's not hard.

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Duskwuff is right, of course. Why go low-level and not use the API?

See http://www.tldp.org/HOWTO/text/IO-Port-Programming section 6.3:

If the device you're talking to supports something resembling RS-232, you should be able to use the serial port to talk to it. The Linux serial driver should be enough for almost all applications (you shouldn't have to program the serial port directly, and you'd probably have to write a kernel driver to do it); it is quite versatile, so using non-standard bps rates and so on shouldn't be a problem.

To illustrate this, here is the answer to the original question "how to set the baud rate":

In addition to the port (e.g. 0x3F8 for COM1), the next few port numbers are used for various controlling purposes. For setting the baud rate, first calculate the divisor 115200 / [desired baud rate]. E.g. if you want 38400 baud, the divisor is 3. Then:

  • Set the highest bit of PORT+3
  • Send the least significant byte of the divisor to PORT
  • Send the most significant byte of the divisor to PORT+1
  • Clear the highest bit of PORT+3 (at the same time, the lower bits can be used to set the parity, stop bits etc.)

In code:

   outb(PORT + 3, 0x80);    // Enable DLAB (set baud rate divisor)
   outb(PORT + 0, 0x03);    // Set divisor to 3 (lo byte) 38400 baud
   outb(PORT + 1, 0x00);    //                  (hi byte)
   outb(PORT + 3, 0x03);    // 8 bits, no parity, one stop bit

Source is here: http://wiki.osdev.org/Serial_Ports

Robin
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