I'm using a STM32f373x and with no battery. The product specification on page 77 says it should be connected to VDD. I have made a test with Vbat pin left open and the micro is working. Why I should connect Vbat to VDD and why it may work if it's open?
STM32 Microcontroller – Why Connect Vbat Pin to VDD When No Battery Is Present
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Correct on both counts. The default mathematical operation in an equation when none is given is multiplication.
Just for anyone that has experienced the same problem I'm posting the solution.
The problem was that the microcontroller was damaged in such a way that the protection diode of some port formed a short circuit between the power supply and the ground:
There were two reasons for that. The first and main reason was that I have unplugged the JTAG cable, while the PCB was switched on. I'm trying to figure out exactly how, but that action made some of the ports acting as a ground and damaging the protection diode to act as short circuit. So the conclusion is first power off then unplug the JTAG. The second reason was that the JTAG was supplied with 3.3V (VCC) from the debugger. For my previous boards JTAG supply was provided always by the PCB itself, but now for some reason it didn't worked out (though I'm measuring correct supply provided TO the JTAG) so I had to switch to the debugger power supply. I think that supplying from the debugger and at the same time from the board made a short the connection between a port and 3.3V supplied from the PCB and burned that diode.
Conclusions:
- avoid using the JTAG debugger supply (VCC) and
- avoid unplugging the JTAG cable, while PCB is powered on or
- use a dc/dc insulator for insulating the JTAG GND and VCC
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Best Answer
There is an excellent video by Dave Jones of the EEVBlog explaining why you can power almost every IC through almost every pin. This applies to your situation as well since you're supplying power through VDD. Vbat will be connected through the ESD protection diodes (which are present on virtually all pins on the IC) to the VDD pin and taking it's supply via this "detour".
It is just good practice not to leave the Vbat pin open but instead connecting it to VDD. Although leaving Vbat open works perfectly well as you have found, the circuits feeding from Vbat will operate at a lower supply voltage because they receive their supply voltage through the ESD diodes. In certain situations this could lead to unwanted effects. For example the minimum supply voltage at which the processor will work might be different.