NUCLEO L476RG with MCP23S17 on SPI communication supplied with +5V not working. When Connecting to 3V3 working, but why?

[JKade.3]

Hello,

we've connected A0-A2 and VSS with ground. VDD and RESET are connected to supply voltage (+5V).

The system does only work with 3V3 not +5V, but we don't see the problem how it's not going to work with the higher voltage.

with regards

Julius

[gregstm]

The maximum voltage of the STM32L476 micro is 3.6V (well, the data sheet says 4.0V Absolute maximum, but you would want to stay clear of that in my opinion). 5V may enter the Nucleo board from the USB, but a regulator brings it down to 3.3V. Perhaps study the schematics in MB1136.pdf

[waclawek.jan · Accepted solution]

@Community member​ ,

@JKade.3​  probably meant that the MCP23S17 is supplied from +5V.

Julius,

The STM32 is supplied from 3V, so its output levels may be not sufficient to drive the MCP23S17 properly. Observing the signals using oscilloscope might be revelatory.

JW

[JKade.3]

Thank you for the replies.

We changed to use the 3V3 supply, so we can use it.