FAQ: STPM32 Remote Reset

Document created by Central Support Employee on Jul 5, 2018
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Q:

 

STPM32 is used in for an e-meter application. We want to isolate only the RX and TX with opto-couplers and do not plan to use the SYN and SCS for reset.
Reading the data sheet (https://www.st.com/resource/en/datasheet/stpm32.pdf) on page 68 it says::

The remote reset can be performed in two ways:

  • Using SYN and SCS pin
  • Writing the reset bit (S/W reset in DSP_CR3)

Does this last option correctly reset the chip, especially the DSP ?
If yes, 2 opto couplers are not needed for the SYN and SCS lines.

Do you confirm?
  

 

A:

In fact, pulses on SYN and SCS are mandatory to correctly reset the chip set.

In your case, there is a way to save 1 opto-coupler by generating SCS from SYN + TXD signals: you put a OR gate with SYN and TXD as inputs, and SCS as output (the OR gate can be replaced also by 2 diodes).

You get the true table below:

TXDSYNSCS
000
011
101
111


 

Then, at each Power-on sequence:
  • Send 3 pulses on SYN (TXD being high)
  • Send 1 pulse on SCS with SYN/TXD combination:

In the example code below, I set SYN to "0", then send 0x00 on TXD, wait a delay, finally set back SYN to "1".

GPIO_ResetBits( GPIO9, SYN_STPM32);
USART_SendData(USART3,0x00);
INIT_WaitMicroSecond(10000);
GPIO_SetBits( GPIO9, SYN_STPM32);

Be sure that at power-on (when EN and 3.3V rise), SCS signal must be high to lock the chipset in UART mode.

 

 

 

(extracted from ST OLS Support Database)

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