V635-AA21 Kinetic Systems 8-Channel, 100 kHz Frequency Counter

The Kinetic Systems V635 is a single-width, C-size, register-based, VXIbus module with eight frequency measurement channels. This counter module can be used to monitor a variety of pulse sources. Its unique circuitry allows the monitoring of a wide range of frequencies without changing any module settings. Differential input circuits with filtering and hysteresis provide high noise immunity. The switching threshold is programmable, and the input voltage should be at least twice the threshold voltage for noise immunity. AC or DC coupling of the differential inputs is programmable on a per-channel basis.

Frequency measurement is armed by software command, and begins when the first "edge" of the input signal is received. The input pulse stream for each channel is sampled during a user-selected observation window. The window period is programmable, and the selection is common to all eight channels. At the end of each window period, 24 bits of data representing the timebase count from the master clock, as well as 18 bits representing the number of whole periods observed are stored in the Current Value Table (CVT) for that channel. If the period of the input pulse stream is longer than the window period, the window remains "open" until one whole period of the input signal is observed. The CVT memory can be read by software at any time, with the data from the latest observation being read. The frequency can be calculated by host computer software using the following formula:

Frequency = whole input periods x clock rate/timebase counts

The V635 can operate in a single-scan or in a continuous mode. The clock rate for the module is programmable to provide a tick rate of 1 MHz or 10 MHz with a clock accuracy of ±0.0001%. The counting accuracy depends on the time base accuracy as well as the monitoring resolution. The longer the observation window, the higher the accuracy. A 10 ms observation window will result in an accuracy of approximately ±0.01% with a 1 MHz clock, and ±0.001% with a 10 MHz clock. A 100 ms window will provide accuracies in order of magnitude better. Programming the clock to 1 MHz allows a measurement down to 0.06 Hz, while a 10 MHz clock increases the resolution by a factor of 10, but makes the lower counting limit 0.6 Hz. An overflow status bit is asserted for that channel whenever the input frequency is below the measurable limit.

A separate input connector is provided for a "health check" signal. The input circuitry can be switched- under program control-from each of the channels to this input, providing a test of the operating characteristics of that channel. Maskable interrupt source bits are set by an overflow from the time base clock counter. An interrupt can be generated by any one or a combination of these bits. The V635 supports both static and dynamic configuration. Access to the data is through memory locations indicated by the Offset Register within the VXIbus Configuration Register set, using A32/A16, D32/D16 data transfers.