STM32F205RB - unused pins

[rick2399]

Posted on February 04, 2015 at 22:37

Hello - I have a few questions on unused pins for this device -

1. We will not use the RTC.　 Can we leave the LSE pins unconnected?　 Or do we tie them to a rail?
2. We will not use the ADC, DAC, sensors etc.　 Can we leave VddA open or do we have to supply Vdd power to it?　 We’re trying to minimize power consumption so if we can leave the ADC, DAC etc. un-powered that would be best.
3. Per RM003 para 29.5 USB connects to PA9, PA11 and PA12 (peripheral).　 Are other GPIOs supported or must we connect to these ports.

Thank you!

[Tesla DeLorean]

Posted on February 05, 2015 at 00:30

We will not use the ADC, DAC, sensors etc.　 Can we leave VddA open or do we have to supply Vdd power to it?　 We’re trying to minimize power consumption so if we can leave the ADC, DAC etc. un-powered that would be best.

You need the analogue supply, the power-on-reset circuit uses it, as does the PLL/VCO as I recall. If you don't want the ADC/DAC, don't enable their clocks.

[rick2399]

Posted on February 05, 2015 at 16:41Thank you Clive1.  So for the LSE - since this is not being used it is otherwise GPIO so leave it floating + disable the LSE by turning off the LSEON bit in the RCC_BDCR.On the ADC/DAC and the power supply - it will then draw a micro-amp then but otherwise is off.  Right?

[Tesla DeLorean]

Posted on February 05, 2015 at 17:03

The LSE and HSE are only turned on by user code, the HSE may be enabled by the System Loader with respect to USB/DfuSe

There are a number of pins used at Reset, there are another sub-set used by the System Loader where it is usually looking for transitions on interfaces where ISP connectivity might be coming from. If you are building a board, a complete review of the Data Sheet would be a prerequisite. If I where paranoid I'd have weak pull downs to ground on pins I wasn't using, I would not bond them to ground. Most of the pins default to a Hi-Z input state.