Should Serial communication occur at standard Baud Rates?

Question

I am interfacing an ATMega8 microcontroller to my PC using a serial to USB converter. The program I use to receive data is MATLAB. Is it strictly necessary for me to send and receive data in standard baud rates for serial communication? Would it be possible for me to send and receive in, say,208333 bps? I'm using AVR programming at the sending end and MATLAB at the receiving end, and I'm wondering why I must use standard baud rates?

I'm using a DKU-5 cable modified to a serial converter in Windows 8.

Answer 1

An RS-232 serial port operates with an implicit clock. The receiver in the USB converter synchronises to the transmitters clock by identifying the middle of the start bit and then samples subsequent bits a single bit timing later. In order to sample the bits in the middle and limit the effect of jitter and timing skew (Asynchronous communication) the receiver typically samples the signal at 16 times the actual data rate. This implies that the receiver is able to produce a clock signal at this speed by dividing its oscillator by an integral number to reach the sampling rate.

The oscillators are typically chosen to allow divisors that produce standard clock speeds with low error rates, particularly at the higher speeds. Choosing a non-standard speed is likely to give to a large error from the desired speed increasing the likelihood of transmission errors.

The classic way (which may not be applicable here) is to use a synchronous link that does not require the oversampling and allows an increased speed. This is probably easiest to implement in your case by introducing a USB slave into your device. This will then support the host clocking that will be 1 Mbit/s, much faster than any asynchronous link.

A more hardware oriented site may give you better answers.