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1 Introduction Frequency measurement is the most basic of electronic measurement technology. Commonly used method of measuring frequency greater limitations, the measurement accuracy is decreased with the measured signal frequency decreases, and the measured signal is generated count ± 1 digit error. The use of other methods of measurement precision frequency measurement accuracy, precision constant;, and with the CPLD (complex programmable logic device) allows the combination of frequency range of up to 0.1Hz ~ 100 MHz, measured the global relative error constant frequency of 1 / 1000.

Intelligent instruments usually microcontroller core, microcontroller their general counter braves white home jersey / timer count / time range or precision sometimes can not meet the system requirements. To 89C51 microcontroller, for example, when its two 16-bit internal counter / timers T0 and braves white home jersey T1 work in the counting mode, on T0 (P3.4) or T1 (P3.5) to count external pulses. T0 or T1 pin when a negative transition occurs, the counter plus 1. As a negative transition identification pin be two machine cycles or 24 clock oscillation cycle. T0/T1 maximum frequency 1/24fosc, when the crystal oscillator frequency is 12MHz, its maximum count frequency of 500kHz. Require high measurement frequency, the measured signal preprocessing is required for the expansion of frequency range.

2 Measurement principle Required measurement frequency is higher, is required for both high and low frequency using different measurement methods to improve measurement accuracy.

2.1 Frequency Measurement Frequency measurement using frequency braves white home jersey measurement. In determining the threshold time Tw, the record number of cycles measured signal change (or pulses) Nx, then the measured signal frequency: fx = Nx / Tw. Frequency Measurement principle of Figure 1. Since the measured high frequency, single chip difficult to measure, 8051 was the highest frequency measured 500kHz, so using CPLD and 8051 the composition of the internal counter 32-bit counter. CPLD lower 16-bit counter, the carry pulse count back to 8051. CPLD delay is 10ns, therefore, CPLD and 8051 according to the frequency measurement principle can accurately measure the frequency of 50 Mz.