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AN INTELLIGENT SERIAL COMMAND INTERFACE

by Tom Napier

Start ı The Hardware • The Firmware ı Sources and PDF

THE FIRMWARE

The PIC16C54 microcontroller runs at a clock rate of 6.144 MHz. The serial input algorithm was conservatively designed so that it would not be confused if it powered up in the middle of a continuous string of irrelevant ASCII characters. It bit-slips through successive random characters until it locks on the start and stop bits. Then it looks for a colon. All other characters are ignored. (The command strings are transmitted as comments so that the other equipment being controlled does not get confused by them. Colons are not used to control the other equipment in the system.)

After finding a colon, the microcontroller starts examining successive characters. If the next character is neither the assigned address character or an "@", the unit switches back to search mode. Otherwise, it starts storing characters and comparing them to the valid range. Any invalid character immediately restarts the colon search. Even the right number of valid characters must be followed by a semicolon before anything further happens. At that point, the selected commands are sent to the relay driver chips via the PICıs output pins.

Whenever a command switches an input from one output connector to another, the firmware turns off the current output relay. It waits long enough for the relay to switch off before turning on the new output. This avoids overloading the signal source. However, the program first checks whether a command will change the output relays. This stops it from switching an output off momentarily (i.e., it avoids output glitches) if the same command is sent twice or if the auxiliary outputs are switched without altering the output relay connection. Listing 1 shows the PIC program.

A UNIVERSAL REMOTE CONTROL

The outcome of this development is a compact remote control receiver, which can be adapted to many other functions with a few changes to the firmware. System size and cost have been reduced by combining the serial interface and the function decoding in a single chip.

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