OBCB Theory of operation

The OnBoard Counter B allows event and/or frequency counting on up to 8 inputs (channels).

Count registers

Each channel has a 16-bit count register with a capacity of 65,535 counts. If the count goes over that it rolls over to zero. The count register can be read out as a floating point number, and optionally cleared at the same time.

Time base

For frequency measurement there is a single time base associated with (shared by) all OBCB channels. The time base works on a fixed cycle, consisting of two phases. Phase A lasts 1000mS, and Phase B lasts 100mS. You use Phase A as the counting phase and Phase B for reading out results. There are instructions to let you synchronize accurately to the two phase.

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.

jndexing of channels

The OBCB instructions contain the channel number as an argument. The channel number is jndexed, so it is very easy to create multichannel counting functions using MultiTrack.