June 2005, Issue 179

Precision Frequency Meter
Cypress PSoC High Integration Challenge 2004 Contest Winner

USER INTERFACE

My three-button frequency meter includes a two-row LCD module. The meter works in two modes: Time Display and Frequency Measurement. In the former, the meter displays the current time (synchronized to the national standard) while monitoring a radio broadcast for time pips. The buttons enable you to adjust the hour (the time pips mark the hour, but don’t indicate which hour) and initiate Frequency Measurement mode, in which the hardware is reconfigured to measure an input frequency. Continually sampling the input, the meter measures the period (in reference clock counts) of a number of cycles over approximately 200 ms. It then calculates and displays the frequency in hertz (to as many decimal places as possible) along with an estimate of the error.

You can select from two input modes in Frequency Measurement mode: Analog and Digital. In the former, the input frequency is fed to a comparator before it’s fed to the digital logic. This enables you to measure low-level signals, but the maximum frequency is limited to approximately 80 kHz. In Digital mode, the input is fed to a digital input, thus providing faster operation, but it requires the signal to be over a few volts peak-to-peak.