Reset line pulled low for no reason

[james.stocktonj1]

I have created my own board based on the STM32L073RZ where I copied certain bits from the NUCLEO borad. I am having a problem where I can't connect to it using the STLINK. I have found out that even with only power plugged in the reset line is pulled low for no reason. I have double checked my schematic and the pinout of the chip. I have also tried adding a 10k to 3V3 but it is stilled low. It can't be that there is a short to ground because I have done a continuity check and they are not connected, the chip itself must be pulling it low. Can anyone help??

[AvaTar]

The problem is either on your side (schematic, layout, PCB), or the MCU is damaged.

If it's not BGA, you can try to desolder the NRST pin, and pull it up.

[james.stocktonj1]

Yes I agree with that, I've made two boards and both of the chips where soldered professionally (I didn't feel like attempting a LQFP64 at home), so its unlikely shes blown both chips or shorted any pins. 'll try lifting the NRST pin but I have a feeling its something inside the chip, the board layout and schematic should be fine (I've double checked them both) but the continuity check should have flagged up if they where connected to ground.

[james.stocktonj1]

Do you know whether the BOOT0 pin would be in any way linked to the NRST pin. What state should BOOT0 be in when I'm booting from the internal flash?

[AvaTar]

See the reference manual, sect. 2.4 (page 64):

BOOT0 = L at the NRST L/H transition means booting/running from internal Flash.

Otherwise, I know of no backward dependency (i.e. BOOT0/1 influencing NRST).

[AvaTar]

As one usually learns the hard way, first prototypes usually contain routing errors.

I would lift up the pin, and measure with and without pull-up.

I'm not a hardware guy, but used to be employed in sparsely populated embedded SW departments, and thus had to get such boards working myself.

[Tesla DeLorean · Accepted solution]

Make sure you have VDDA powered​, otherwise it will clamp NRST low in the POR circuit.

[james.stocktonj1]

Thank you very much for that. It is now fixed and I can upload to the chip now. I didn't really think that the VDDA power would matter, I tried putting as little components on the board first as possible.

Thanks a lot.

James