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On the STM32F103C8T6 there is a 5V pin, see at the bottom left of the STM:

enter image description here

However, in the official description from ST.com (see Link), in the table on page 28 is shown:

C2 F2 - - - 10 - VSS_5 S - VSS_5 - -

Which means the VSS 5V pin is a Supply pin. However, does this mean I can power the STM with a (USB) power adapter and connect the 5V power to this pin, or is it only for powering components connected to the STM with 5V?

Note, there are also VSS_4, VSS_3, VSS_2 and VSS_1 pins, so I'm even wondering if the VSS_5 pin is 5V, and which pin maps with with +5V on the picture.

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That is a board with an stm32 processor on it, known as a blue pill, rather than an stm32 as a product.

The schematic for the board is http://wiki.stm32duino.com/images/c/c1/Vcc-gnd.com-STM32F103C8-schematic.pdf

The 5V pin you mention is pin 3 of the header P4, connected to a net called 5v. There is then a regulator to bring the 5 V down to 3.3 V on the VCC3V3 net which is used for powering the processor, so yes, you can connect 5 V there, or use the USB socket to power the processor. The pin labelled 5V and the USB power pin are shorted together, so you should only drive one of them.

The schematic seems to use VCC3v3 and VCC3V3 interchangeably, and the same with 5v and 5V, I'm assuming they're the same nets, else the regulator is redundant.

You have a misunderstanding about pin VSS_5 from the datasheet, that's saying it's the 5th VSS pin, and should be grounded. All the VSS pins should be connected together, and connected to ground, there are many VCC and VSS pins around the processor to help with power distribution.

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