OBCA: Theory of operation

The OBCA consists of:

• A timebase generator which can be set for 0.1S, 1S or 10S,
• A 16-bit hardware counter (capacity 65,535 counts). This counts incoming pulses from a physical input.
• An 8-bit Firmware extension to the counter, which extends the count range in totalizing mode to 16,777,215 (24 bits).
• A result register that your program can read into floating point register W and
• A reading available flag that your program can test.

In frequency mode, at the end of each timebase interval, the counter contents is transferred to the result register, the counter is cleared and the flag is set. When the result is read out the flag is cleared again. This allows you to make sure you only process fresh readings. If you do a readout when the flag is clear, you will simply get the previous reading.

The accuracy of the readings will depend on timebase error and quantisation error. Typically this is:

• For controllers that use a crystal for timing: 0.1% plus/minus 1 count.
• For controllers that do not use a crystal for timing: 2% plus/minus 1 count.

In totalizing mode the counter simply accumulates counts until reset.