piclist 1998\10\07\051418a >
Thread: 4 digit auto-ranging frequency-meter.
face BY : Regulus Berdin email (remove spam text)

Peter Gee wrote:
> What would happen in the schematic shown if the input voltage when negative
> with respect to the gnd of the PIC ?
The protections diodes may protect it from damage but excessive current
may destroy it ultimately.

> my guess is that it wouldnt be good for the PIC.
> in a similar project ( from memory), they use a coil, capacitor, signal
> diodes, and a transistor to filter and switch the incoming frequency to make
> it nicer for the PIC to count.
The posted circuit is incomplete, for a more sensitive frequency
counter, it needs a pre-amplifier and convert its output to TTL level.

> i built the project, and it seemed to give accurate results in comparison to
> a bench frequency counter at a frequency of 40Mhz using a 10Mhz 16cf84.  (i
> used an fm RC transmitter to calibrate it) -- i don't have a frequency
> generator higher than that so i can't test its high end response.
The PIC doesn't need to be 10MHz, the input is captured in the RTCC
using it's prescaler which is rated at 50MHz. The pre-scaler cannot be
read directly, that is why RB0 is connected at RA4 to self clock the
pre-scaler to get it's value.  The 470 ohm resistor is to prevent
contention from the input and RB0.


<361B2541.F4ACAC2@skyinet.net> 7bit

See also: www.piclist.com/techref/index.htm?key=digit+auto+ranging
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