Hello Jan,

Visit this site http://www.edn.com/archives/1998/030298/05di.htm
for the complete story about how to use a MCU to measure high speed. It will explane why a second gate is used.


The second gate is used to extract the residual left over in the 74LS393 after the timing period (1 second) has expired. You toggle the input to the 74LS393 until the most significant bit goes low which is sensed by the interrupt Zero edge triggering interrupt. You simply add the amount of toggles to the other registers to get a total count of frequency. By the way that part of the software is not incorporated yet but will be soon.

To read the value of the low 12 bits of the frequency, the µC toggles CNTL2 (second 74LS00) N times until the internal 8-bit timer increments by one. The low 12-bit value of the frequency is then equal to 4096­N. Reading the value of the counter in this manner requires only one I/O line as opposed to 12 lines to read the counter's 12 bits. In combination with the internal 8-bit counter in the µC, a 20-bit frequency measurement is possible. The limiting factor of the measurement is the maximum frequency input to the NAND gates and the ripple counter. The input frequency can be extended to 125 MHz by using a 74F00, and two 74F393's.

A good inductance and capacitance meter can be built with a LM311 comparator and using this frequency counter. For a really good explanation with equations and circuits visits this site. http://www.qsl.net/iz7ath/we.../index.htm

The latest hex listing can be seen at this address.
http://members.aol.com/cpu8052/4051fc.txt This listing incorporates software to drive the MAX7219 LED display. The 89C4051 had to be used because the program size exceeded the 89C2051 size limitations (2048 bytes flash).

Hope this answers the question.

Regards,

Charles Bannister