I always found that circuit (and other 16F84 freq counter circuits) kind=20
of odd.
There is no need to toggle the RTCC input to increment the pre-scaler. =20
You only need to stop the pulses.  Once the pulses stop, you can toggle=20
the internal T0SE (Timer0 edge select) bit to increment the pre-scaler,=20
no hardware is required!  You'd think that MicroChip would know that,=20
but they did it the hard way and everyone else followed suit.

Kerry

On 6/23/2014 7:25 PM, pc wrote:
> Did you understand the gating circuit?
> Is it of no help?
>
> For your case only 1 (U2a) not all 4 nand gates would be needed,
> The other three enable RA2 to toggle the rtcc internal prescaler till it
> rolls over
> This enables a pic running with a 4Mhz clock to count to 50Mhz update at
> almost 10hz with 10hz resolution
>
> http://www.piclist.com/techref/piclist/weedfreq/circuit.htm
>
> PC
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf =
Of
> Richard R. Pope
> Sent: 24 June 2014 00:32
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT]Building a Freq Meter!
>
> PC,
>       I appreciate the information but I am trying to build an eight
> digit seven segment LED frequency counter. I have been able to design
> all of the elements except for the gating circuit. I am kind of
> committed because I have already built the display board with the CD4511
> driver chips.
> Thanks,
> rich!
>
> On 6/23/2014 6:13 PM, pc wrote:
>> I posted this a while back
>> Simple to build
>> PC
>>
>> http://www.piclist.com/techref/piclist/weedfreq.htm
>>
>> -----Original Message-----
>> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of
>> Richard R. Pope
>> Sent: 23 June 2014 07:22
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT]Building a Freq Meter!
>>
>> Hello all,
>>        I hope that I can get some help. I am attempting to build a
>> frequency counter. I have the input amplifier, counter circuits, and the
>> time base built. I am having trouble building a gating circuit. The
>> input consists of a diode limiter, 2n4401 amplifier, a capacitor to
>> eliminate any dc offset, and a 74hct14 schmitt inverter to condition the
>> signal.         The display is eight CC seven segment LEDs driven by
>> eight CD4511 BCD to seven segment latching decoder drivers.
>>        The timebase is a four MHz 2n3904 crystal oscillator that is
>> amplified by a 2n4401 transistor and then fed to a 74HC4060 14 stage
>> binary ripple counter. This conditions and steps the 4 MHz signal down
>> to 250Hz. I then feed this to two 74LS390 decade counters. This gives me
>> 10Hz, 5Hz, 2Hz, and 1Hz signals. These are fed to a 74HC4053 triple dual
>> channel multiplexer.
>>        These are connected so that I can turn the signal off which would
>> allow the counter to run in free running mode or to select one of the
>> four frequencies of 10, 5, 2, or 1Hz for controlling the gate timing.
>>        I can't come up with a gating circuit that will provide the
>> necessary trigger window, the latching signal, and the reset pulse.
>> Could someone please help me?
>>         I also thought about using a PIC to supply the time base, the
>> reset pulse, the gating window, and the latching signal, but I don't
>> even know where to begin but I am sure that it could be done pretty
>> easily with the PIC. That would eliminate the timebase, the dividers,
>> and the switch. One chip would replace two transistors, two 74ls390s,
>> and a 74HC4052 multiplexer. Does anyone have any ideas or thoughts on
>> any of this?
>> Thanks,
>> rich!

--=20
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