We have a production issue where about half of our units have odd inconsistent behavior. Failures vary from board-to-board - DFU failure, BLE connection issues, partial initialization, fully unresponsive, etc. Some units appear to operate normally.
We recently identified a PCBA design issue which can allow 12V directly into AIN0. Is it possible that this Overvoltage event could sometimes cause system-level failures?
Not only possible but in time virtually certain although the faulty behaviour might take time to manifest itself. The reason is the external voltage turns on an internal schottky protection diode which connects the external 12V to the internal nRF52840 VDD and that high voltage will break down internal circuits; worse this high voltage will also bleed out on other nRF52840 pins into other external circuits.
I shared this circuit I produced to illustrate those internal schottky protection diodes connected from the AIN pins to internal VDD (indeed all io pins):
// RTD Measurement - 4-wire Reversible current option // + ----------------------------------------------------------------------+ // | nRF52832/nRF52833/nRF52840 | // | | // | VDD VDD VDD | // | --#-- --#-- --#-- | // | | | | | // | __|__ | Excitation | Option | // | / \ +-| On +-| 13k | // | /-+-\ |<- H0 H1 |<- Pullup | // | | +-| 160R 35R +-| (not used) | // Excitation P0.04| | | | | // +-----------------O-----#------#----------#-------------------#------------ In | // | | | | | Excitation | Option | // | | __|__ ----- +-| Off +-| 13k | // | | / \ ----- |<- L0 L1 |<- Pulldown | // | | /-+-\ |3pF +-| 160R 35R +-| (not used) | // | | | | | | | // | | ===== ===== ===== ===== | // | | === === === === | // | | = = = = | // | | | // | | VDD VDD | // | | --#-- --#-- | // | | __|__ Auto-adjust: | 160k | // | | / \ Bias inputs at VDD/2 +-| Bias Hi/Lo/Both | // Red Excitation A | | /-+-\ VDD1_2 |<- | // +--------/~/--------+ | | PIN_VOLTAGE_1 +-| | // | White Sense A P0.28 | | AIN4 |----'T'-Mux-P---| | P SAADC | // #--------/~/------------------#-------O-----#------#-------#--+ +----#-------+ +--#-----#------#------#--> | // | | | | | | | | | | | | | | | | // | | | __|__ ----- | ---- +-| ----- | +-| +++ | | // | | | / \ ----- | | |<--+ | | |<- | | ----- | // +++ 100nF | | /-+-\ | 3pF | | +-| | | | +-| | | ----- | // | | ----- | | | | +---- |--------# | | +++ | 2.5pF | // | | ----- | ===== ===== | | | | | | |1M0 | | // +++RTD | | === === | ===== | | | ===== ===== ===== | // | Remote | | = = | === | | | === === === | // | | | | = Select | = = = | // | | | | | | // | | | | | VDD | // | | | | |----'T'-Mux-N---| | --#-- | // | | | +--+ +----#-------+ +----+ | 160k | // | | | | | +-| Bias Hi/Lo/Both | // | | | PIN_VOLTAGE_2 | | |<- | // | White Sense B | P0.29 | AIN5 |----'T'-Mux-----| | | +-| | // #--------/~/------------------#-------O-----#------#-------#--+ +----#-------+ +--# | | N SAADC | // | | +--+ +----#-------+ +----#---#------#------#--> | // | | | | | | | | // | | PIN_VOLTAGE_3 | | +-| +++ | | // | P0.31 | AIN7 |----'T'-Mux-----| | | |<- | | ----- | // +--------/~/--------#---------#-------O-----#------#-------#--+ +----#-------+ +--# | +-| | | ----- | // Red Excitation B | | | +--+ +----#-------+ +----# | +++ | 2.5pF | // | | | | | | |1M0 | | // | | | PIN_VOLTAGE_4 | | ===== ===== ===== | // | | P0.30 | AIN6 |----'T'-Mux-----| | | === === === | // | | +---O-----#------#-------#--+ +----#-------+ +--# | = = = | // | | | | +--+ +----#-------+ +----# | // | 100nF | | | VDD | | | // | | | | --#-- | | P: Single-ended | // | | | | | | | P&N: Differential | // | | | O-----# PIN_VOLTAGE_0 | | | // Rref +++ | | | | AIN_VDD |----'T'-Mux-----| | | | // 2k0 | | ----- | | +-----#--------#--+ +----#-------+ +--+ | | // 1% | | ----- | | +--+ +----#-------+ +----# | // (0.1%) +++ | | | | | // | #---+ | GND |----'T'-Mux-----| | | // | | O-----#-----------------+ +----#-------+ +----+ | // | | | | | // | | | ===== | // | | | === | // | | | = | // | | | | // | | | VDD VDD VDD | // | | | --#-- --#-- --#-- | // | | | | | | | // | | | __|__ | Excitation | Option | // | | | / \ +-| On +-| 13k | // | | | /-+-\ |<- H0 H1 |<- Pullup | // | | | | +-| 160R 35R +-| (not used) | // | | | | | | | // +---------#-------O-----#------#----------#-------------------#------------ In | // P0.03 | | | | Excitation | Option | // | __|__ ----- +-| Off +-| 13k | // | / \ ----- |<- L0 L1 |<- Pulldown | // | /-+-\ |3pF +-| 160R 35R +-| (not used) | // | | | | | | // | ===== ===== ===== ===== | // | === === === === | // | = = = = | // +-----------------------------------------------------------------------+
