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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