Wednesday, November 6, 2013

PIC Serial Communication Tutorial (UART)

First, a quick history of RS232. What is RS232? It's just a name for a standard that has propagated from generation to generation of computers. The first computers had serial ports that used RS232, and even current computers have serial ports (or at least USB ports that act like RS232 ports). Back in the day, serial information needed to be passed from devices like printers, joysticks, scanners, etc to the computer. The simplest way to do this was to pass a series of 1s and 0s to the computer. Both the computer and the device agreed on a speed of information - 'bits per second'. A computer would pass image data to a printer at 9600 bits per second and the printer would listen for this stream of 1s and 0s expecting a new bit every 1/9600 = 104us (104 micro-seconds, 0.000104 seconds). As long as the computer output bits at the pre-determined speed, the printer could listen.
Zoom forward to today. Electronics have changed a bit. Before they were relatively high power, high voltage devices. The standard that is 'RS232' dictates that a bit ranges from -12V to +12V. Modern electronics do not operate at such high positive and negative voltages. In fact, our PIC  runs 0V to 5V. So how do we get our 5V micro to talk the RS232 +/-12V voltages? This problem has been solved by the IC manufacturers of the world. They have made an IC that is generically known as the MAX232 (very close to RS232, no?).
The MAX232 is an IC originally designed by a company called Maxim IC that converts the +/-12V signals of RS232 down to the 0/5V signals that our PIC  can understand. It also boosts the voltage of our PIC  to the needed +/-12V of the RS232 protocol so that a computer can understand our PIC  and vice versa. To get our PIC  IC sending serial characters to a computer, we have to send these serial signals through a MAX232 circuit so that the computer receives +/-12V RS232 signals. Don't worry if you're working with a chip labeled 'ICL232' or 'ST232' - these are just generics of the MAX232. Everyone says 'MAX232'. The ICs all function the same and nearly all have the same pinout.

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