When bringing up a new hardware design, sometimes it happens that a connection gets overlooked, or on a prototype, a bad solder joint or a short prevents some pin being physically connected to where it belongs. This may result in a situation where "nothing is working", i.e. the STM32 does not respond to a known-good programmer or debugger.
Here is a list of pins, which should be connected in some particular way for basic functionality of an STM32 design:
- all VDD/VSS pins must be connected to a stable power source providing voltage within specified range (usually 1.8V..3.3V) and properly decoupled.
- VDDA/VSSA pins must be connected to specified power supply too. Some STM32 models (probably all 'Fxx families) require VDDA to be within a relatively small difference from VDD; this is important especially during powerup, so the VDDA circuitry must be designed with this in mind (for what may happen if this is not maintained see e.g. here, here and here ).
- VCAP, if present, must be connected to properly sized capacitors as given in datsheet; if there are several such pins, this applies to all of them.
- PDR_ON and BYPASS_REG, if present, must be connected as given in the datasheet.
- leaving VBAT unconnected may be asking for additional trouble, althought the VBAT brownout erratum requires software intervention (performing backup domain reset upon startup), even if the otherwise unused VBAT is connected to VDD as specified in DS.
- BOOT0 if present; in newer STM32 this may not be needed if relevant option bits are set so that BOOT0 pin is ignored (if booting from System memory is required to use the built-in bootloader, BOOT1 pin, if present, must be appropriately connected, too).
- contrary to popular belief, it's not absolutely necessary to have any special circuit for NRST due to built-in pullup; although a decoupling capacitor is recommended by the DS. However, NRST must not be driven actively high by external circuit, otherwise internal reset sources won't work.
For each STM32 family/subfamily, ST does provide an appnote named Getting started with STM32xxxx MCU hardware development or similarly. These do contain most the above information and more, so it's essential to check them out before starting to make a new hardware design. Another good source of inspiration for hardware design are be the Nucleo/Disco/EVAL boards, for which ST provides schematics and BOM on their respective product pages.