We're bringing up yet another custom board with the STM32H735 as the core processor. Built the first one, handed it to the software guy, and a few minutes later he says it's broken. Won't program, won't go to DFU mode, nothing. "You must have zapped it by accident!" Grumble.....get out the hot air gun and swap the part. At least it's a QFN100 and not some BGA package. Fixed, dropped it on his desk. Go to lunch.
Phone rings. "It's broken again. As soon as I downloaded code it died." Aaarrggghh! Back from lunch, steam off processor #2, install #3. This time, I'll make sure. Create a new IOC file, populate it with all the right pin settings, configure the peripherals, generate code in CubeMX, open project in CubeIDE. So far, so good. Add a couple lines to flash the debug blinkylight, hit download. Code downloads, processor goes away. No blinky. No nothin'.
Vcore is at zero. On power-up, but NOT after hardware reset, there's a momentary blip on Vcore followed by a decay to ground. Somehow-or-other, the power supply setting was set for SMPS....and we're not using the SMPS. All of that is grounded. We had to go searching for this setting, and neither of us knew where to find it so it's a mystery how it got set that way. It should have been set to PWR_LDO_SUPPLY, which enables the internal LDO for Vcore and disables the SMPS.
Once you've done this, there's no way back without finding a way to power Vcore externally, or scraping the part off the board and replacing it. In my case, I grabbed a LM117 adjustable regulator and tacked the adjust pin to ground and the supply to 3.3V. That sets the output to 1.25V, which I could then hotwire to the Vcore cap. Blinkylight! Hurray!
So.....if you're not using the SMPS, and you don't have an external source for Vcore, it's critical to make sure that the power supply is set to PWR_LDO_SUPPLY. You'll find this in CubeMX under the RCC settings. That is not where I would have expected to find it, and we had to fish around for a while before we tripped over it.
Seems to me that this should always default to LDO_SUPPLY, and there should be some kind of warning that brickage could happen if you don't have an external Vcore supply hooked up and any other option is selected. Fortunately I was careful pulling off the bricked parts, so I can re-use them and tack on my temporary hack to get them erased so they're back to working again.
