Configuring a Counter to Measure Frequency (Scan Interface)

You can configure a counter to measure the frequency of a signal connected to the channel. The counter counts the number of rising or falling signal edges during the length of the Frequency Timebase, divides that number by the Frequency Timebase in µs, and returns the frequency in kHz. You can configure the following options for a frequency measurement counter.

Configuring the Input Filter

You can add an input filter to all input channels of the module. A filter removes noise, glitches, and spikes on inputs by rejecting signals with periods shorter than the specified length. You can select one of the following options.

• Disabled—The hardware synchronizes the digital inputs using a 4 MHz timebase. Pulses longer than 250 ns are always detected. Pulses shorter than 250 ns may be rejected.
• 1 µs—Pulses shorter than 1 µs are rejected. Pulses between 1 µs and 2 µs are sometimes detected. Pulses longer than 2 µs are always detected.
• 16 µs—Pulses shorter than 16 µs are rejected. Pulses between 16 µs and 32 µs are sometimes detected. Pulses longer than 32 µs are always detected.
• 256 µs—Pulses shorter than 256 µs are rejected. Pulses between 256 µs and 512 µs are sometimes detected. Pulses longer than 512 µs are always detected.
• 4096 µs—Pulses shorter than 4096 µs are rejected. Pulses between 4096 µs and 8192 µs are sometimes detected. Pulses longer than 8192 µs are always detected.

Selecting the Measurement Edge

• Rising—The counter counts rising edges.
• Falling—The counter counts falling edges.

Selecting the Frequency Timebase

Selecting a Frequency Timebase is a tradeoff between resolution and response speed. A shorter timebase provides lower resolution but the data it returns is more up to date. A longer timebase provides higher resolution but may lag behind or miss changes in signal frequency.

Note  For frequencies lower than 1 kHz, configure a counter to measure period and calculate the frequency using the inverse of the measurement.