[time-nuts] Frequency Counter using OCXO and MCU

[Chris Albertson]

 Sorry forgot to add this.

As for delayed turn on.  That can work but why not simply have the software
go into a 5 or 10 second wait before it does anything else.  Display
"warming up" or "please wait" on the LCD.


On Wed, Mar 12, 2014 at 7:18 AM, Chris Albertson
<albertson.chris at gmail.com>wrote:

>
> Are you putting the "unknown signal" to be measured on an interrupt pin?
>  that will work for low enough frequencies but most uPs have a built-in
> counter.   It is a hardware register on the uP chip that will increment for
> each pulse on a pin.  then you read that number and divide by the gate
> time.   At some point the frequency will be to high for the counter pin so
> then you switch in a hardware frequency diver as a pre-scaler.
>
>
> On Tue, Mar 11, 2014 at 8:24 PM, d0ct0r <time at patoka.org> wrote:
>
>>
>> Hello,
>>
>> I am experimenting to build frequency counter using external OCXO and
>> ST32 MCU. The OCXO is external DATUM 2750013-1 device which produce 10Mhz
>> sine wave. I connected its output to OC_IN on MCU. I have few challenges
>> now.
>>
>> First, looks like I need to create some delay to turn on MCU _after_
>> OCXO. If I try to start both devices simultaneously, I got following result
>> for 10 kHz TTL measurement:
>>
>>
>> System Core Clock: 168000000 Hz
>> SYSCLK_Frequency PCLK1_Frequency PCLK2_Frequency
>> 16000000         16000000        16000000
>>
>> # Starting SuperLoop...
>> FREQ: 105197
>> FREQ: 105263
>> FREQ: 105263
>> FREQ: 105263
>>
>>
>> As soon as I push reset button on MCU, I got correct results for its
>> clocks and correct value for the counter:
>>
>>
>> System Core Clock: 168000000 Hz
>> SYSCLK_Frequency PCLK1_Frequency PCLK2_Frequency
>> 168000000        42000000        84000000
>>
>> # Starting SuperLoop...
>> FREQ: 10019
>> FREQ: 10019
>> FREQ: 10019
>> FREQ: 10019
>> FREQ: 10018
>> FREQ: 10019
>>
>>
>> Another challenge is the fact, that if I increase the input signal
>> frequency, then performance of the MCU decreased. In the other word, I need
>> to wait much more time to have a result. Probably MCU is super busy to
>> handle the interrupt. Say for 10 kHz range its pretty fast. Then for 1 mHz
>> its much slower.
>>
>> Here is main loop:
>>
>> while (1) {
>>         if(j++ < 0xF00000) {
>>             accum += deltaREF; // Moving Average
>>             accum = (accum >> 1);
>>         } else {
>>             uwTIM1Freq = (uint32_t) SystemCoreClock / accum;
>>             printf("FREQ: %ul\n\r", uwTIM1Freq);
>>             accum = j = 0;
>>         }
>>     }
>>
>> The counter is based on timer in "input capture" mode and driven by
>> interrupt:
>> [ See STM32F4xx_StdPeriph_Examples\TIM\TIM_InputCapture ]
>>
>> Also this counter shows incorrect results for low frequency. For example,
>> for 100 Hz:
>>
>> FREQ: 4968
>> FREQ: 5030
>> FREQ: 5056
>> FREQ: 4916
>>
>> I would be interesting to hear any advise how to improve it.
>>
>> And another question is: what will be "pros" and "cons" to transform
>> 10Mhz sine to square to feed MCU ? I tried it, but didn't catch any
>> difference.
>>
>> Here is schema
>> http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_
>> Ideas/Sine-to-Square_Wave_BJT_Converter_Wenzel.gif
>>
>>
>> --
>> WBW,
>>
>> V.P.
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>>
>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
>



-- 

Chris Albertson
Redondo Beach, California