Figure out why POX sensor wont turn on without a USB cable plugged in

drewsum / pulse_oximeter

An open source device for measuring a user's pulse and blood oxygen saturation

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Figure out why POX sensor wont turn on without a USB cable plugged in #8

Closed drewsum closed 4 years ago

drewsum commented 4 years ago

This is odd behavior, and the two shouldn't be related

drewsum commented 4 years ago

This has something to do with POS1P8_PGOOD not going high when the LDO is turned on without a USB cable plugged in

drewsum commented 4 years ago

the application waits for POS1P8_PGOOD to rise during LDO turn on. The LDO properly turns on when the run pin is asserted, but PGOOD does not rise

drewsum commented 4 years ago

Another thing I notice in hardware is that USB UART RX will sit high when the circuit operates properly, and sit low when the circuit misbehaves. USB voltage definitely has something to do with this

drewsum commented 4 years ago

I believe this issue stems from using global +3.3V for VCCIO on FT234XD. This was the first time you tried this, and you shouldn't do this anymore. I think this creates a strange diode clamping this on USB UART RX when USB VBUS is not applied. If I disconnect +5V_USB from VBUS by swiveling L12091, and I rewire +3.3V to +5V_USB, this issue is resolved!

drewsum commented 4 years ago

I believe the resolution to this is to not populate L1201, and instead run a modwire from +3.3V to +5V_USB somewhere on the board. This is hacky and stupid, but it works