Questions about this circuit and a solution

[Lyu.1]

Hi,Master:

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​Platform: STM32F301K8U

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As shown below in Figure 1, PA7 (MCU_14) is a video signal (As shown below in Figure 2), which contains the line sync signal (A area in Figure 2, fixed frequency, there may be noise)​and the field sync signal(B area in Figure 2). I want to distinguish the line sync signal and the field sync signal . My question is as follows:

Question 1: What is the purpose of PA4 in Figure 1 through capacitor to ground and PA2, PA3 short circuit?

Question 2: I want to use the comparator to compare PA7 and DAC1_CH1 and then use the input capture to measure the period to distinguish between the two signals (As shown below Figure 3). Is it feasible? I worry that if there is a lot of noise on the PA7, it may be difficult to distinguish. Is there a better solution? Ask the master for advice.

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Thank you very much!​

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Figure 1

​ Figure 2

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Figure 3

[Tesla DeLorean]

Guess you'd need to ask the guy who designed the board you're reverse engineering.

PA2/PA3 could be configured as IN/OUTPUT, or connected to other circuit nodes. Could be used to detect board/circuit specifics, or simply work out limitations of connecting signals not supported internally.

[waclawek.jan]

Shorting PA2-PA3 shorts inverting input to output of OPAMP1, making a voltage follower out of it. Capacitor on PA4 filters output of DAC. This all of course assuming appropriate setting of GPIO and given OPAMP and DAC.

JW

[Lyu.1]

Hi,waclawek

Thank you for your professional answer.

I looked at this MCU (STM32F301K8U) data sheet, there is no OPAMP1 amplifier, only OPAMP2 amplifier.

[Tesla DeLorean]

Your diagram shows PA2 as a potential input to COMP2 or an output from it.

TIM2_CH3, TSC_G1_IO3, USART2_TX, COMP2_OUT, TIM15_CH1, EVENTOUT, ADC1_IN3, COMP2_INM 

You should look at what PA3 is routable too.

TIM2_CH4, TSC_G1_IO4, USART2_RX, TIM15_CH2, EVENTOUT, ADC1_IN4 

[waclawek.jan]

I'm not going to check but IMO it's pin compatible with 'F303 of the same package, which does have OPAMP1. And then plans may have changed and those pins remained unused.

Or anything else.

As Clive said, you should ask the guy who designed the board.

JW

[Lyu.1]

Okay, thank you very much for your guidance!