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?
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:
(extracted from ST OLS Support Database)