STM32L151c8t6 PCB design problems

[bakhti]

hi we haveseveral boards in need of the stm32L151C8t6 and we have some stock of the mcu in question the problem is that all the pcbs 'from JLCPCB' have this weird issue were the mcu is stuck and i have to manually poke thenrst pin multiple time for it to run normally "poke as in pull it to gnd for it to reset " please i need help with this i'll share some the the designs

example :

example 2:

 

here is an example :

• made a dac output from 0 to max
• start the program nothing happens
• try to jump the nrst multiple times until it starts
• using stm studio and changing a value i in the program so i can see where it stopped
• video explaining

 https://youtu.be/QQdRiFgf2KA

[gregstm]

I see many things I dislike about that PCB design - but here are some things to start-

• the NRST line should have 100nF capacitor connected to ground (as stated in the data sheet), as close to the chip as possible.
• The Boot0 pin is left floating, it needs a 10K pull down (or similar value) to ground, ideally close to the chip. Surprisingly the data sheet doesn't seem to mention this explicitly (it should).

Some more general comments-

• there is no ground plane, even with a 2 -layer board this can be approximated by using the bottom layer mainly as ground - at least under the micro.
• decoupling capacitors should be placed close to the micro near every VD pin with close attention to minimising the resistance/inductance to the ground plane
• try to avoid routing tracks under the micro

If you are trying to do sensitive analog measurements, you might have trouble with noise with this design. You might want to find someone experienced with low noise PCB design.

[DDona.1]

Most likely as mentioned previously Boot0 pin is the main problem, followed by non existent decoupling capacitors. Floating RST pin usually not such big deal while PCB is not next to EMC noisy environment.

I just don't understand why someone would do 2 layer PCB while for prototypes at JLCPCB the price is almost the same as 4 layer PCB.

[bakhti]

i tried fixing the pcbs with all what was suggested in the datasheet including the NRST pull and BOOT0 pull but still nothin works

we have other boards that have these requirements and they won't work either

[raptorhal2]

To add to gregstm:

SWCLK and SWDIO need 10K pullups, else noise could put the unit in an indeterminant state.

Cheers, Hal