3 ms·

I can't really even understand what the defect is - what kind of projects does it affect? I'm only interested in fairly basic applications of the rpi but I don'

I can't really even understand what the defect is - what kind of projects does it affect? I'm only interested in fairly basic applications of the rpi but I don't have the technical chops to know if I'm setting myself up for failure...

axoltl 2y ago
So normally when you have a microcontroller pin and you configure it as an input, you expect it to SINK current (as in, take voltage 'in'). The bug is that if the external voltage is between 1V and 2.5V (I don't remember the exact voltages, don't quote me on that) the pin will SOURCE current, acting almost as if you'd set it to be an output pin. It's not a lot of current, but it's enough to hold the pin at 2.2V. This happens on all microcontrollers btw. Random charge accumulates and pushes the voltage on your pin to some arbitrary point. The way you fix this normally is by providing a path for that charge to escape in the form of a pull-down resistor. Usually you need something in the 100k range. Because of this bug you need something more in the 5k range. For some circuits that's fine, for others it's more problematic.
- LeifCarrotson 2y ago
  > This happens on all microcontrollers btw. Random charge accumulates and pushes the voltage on your pin to some arbitrary point. The way you fix this normally is by providing a path for that charge to escape in the form of a pull-down resistor. Usually you need something in the 100k range. Because of this bug you need something more in the 5k range. Many microcontrollers provide an internal, selectable pull-up or pull-down resistor (or neither). For example, on the STM32, it's a 30-50k, and individually selectable on a per-pin basis: https://www.keil.com/dd/docs/datashts/st/stm32f10xxx.pdf#G11.1154906 https://www.keil.com/dd/docs/datashts/st/stm32f10xxx.pdf#G11... It's normal for pins to float, it's not normal for them to both lack an internal bias option and to float to a condition where they source significant current. You don't typically put a very low impedance external pull-down resistor on every single input pin.
- moefh 2y ago
  > when you have a microcontroller pin and you configure it as an input, you expect it to SINK current (as in, take voltage 'in'). That's not how it works; input does not mean sink current. An OUTPUT low sinks current, and an output high sources current. In a "normal" microcontroller (with no silicon bugs), an input pin is in a high-impedance state ("Hi-Z"), meaning it doesn't sink or source current -- but it can be configured to have an internal pull-up or pull-down resistor, in which case it will (respectively) source or sink a little bit of current, enough to keep it at high or low voltage (i.e., enough for a logical high or low) unless there's something else driving it. The problem with the RP2350 is that (under some circumstances) there's a current leakage between the pin and the voltage rail, so when a pin is configured as input with a pull-down resistor, the voltage will not go down to the low level it needs to read a logical low as expected: it will be at around 2.2V, which is in the "undefined" region.
  - axoltl 2y ago
    You are, of course, 100% correct. In my haste to explain the 'sourcing' behaviour of the errata I accidentally jumped to the sinking verbiage. Input pins are "pressure gauges", not "flow meters".
crest 2y ago
Simplifed when used as digital input a GPIO pin has three voltage ranges: 0V up to the logical low threshold, VCC down to the logical high threshold, and an and undefined range in the middle. The RP2350 burns a lot of power while in this middle range when any proper MCU shouldn't, because its quite common for this to happen. The "cure" is almost as bad as the problem because to stay out of this range you have to apply a strong external pull-up/pull-down resistor. This too wastes power, just not quite as much.

axoltl 2y ago

So normally when you have a microcontroller pin and you configure it as an input, you expect it to SINK current (as in, take voltage 'in'). The bug is that if the external voltage is between 1V and 2.5V (I don't remember the exact voltages, don't quote me on that) the pin will SOURCE current, acting almost as if you'd set it to be an output pin. It's not a lot of current, but it's enough to hold the pin at 2.2V. This happens on all microcontrollers btw. Random charge accumulates and pushes the voltage on your pin to some arbitrary point. The way you fix this normally is by providing a path for that charge to escape in the form of a pull-down resistor. Usually you need something in the 100k range. Because of this bug you need something more in the 5k range. For some circuits that's fine, for others it's more problematic.

LeifCarrotson 2y ago
> This happens on all microcontrollers btw. Random charge accumulates and pushes the voltage on your pin to some arbitrary point. The way you fix this normally is by providing a path for that charge to escape in the form of a pull-down resistor. Usually you need something in the 100k range. Because of this bug you need something more in the 5k range. Many microcontrollers provide an internal, selectable pull-up or pull-down resistor (or neither). For example, on the STM32, it's a 30-50k, and individually selectable on a per-pin basis: https://www.keil.com/dd/docs/datashts/st/stm32f10xxx.pdf#G11.1154906 https://www.keil.com/dd/docs/datashts/st/stm32f10xxx.pdf#G11... It's normal for pins to float, it's not normal for them to both lack an internal bias option and to float to a condition where they source significant current. You don't typically put a very low impedance external pull-down resistor on every single input pin.
moefh 2y ago
> when you have a microcontroller pin and you configure it as an input, you expect it to SINK current (as in, take voltage 'in'). That's not how it works; input does not mean sink current. An OUTPUT low sinks current, and an output high sources current. In a "normal" microcontroller (with no silicon bugs), an input pin is in a high-impedance state ("Hi-Z"), meaning it doesn't sink or source current -- but it can be configured to have an internal pull-up or pull-down resistor, in which case it will (respectively) source or sink a little bit of current, enough to keep it at high or low voltage (i.e., enough for a logical high or low) unless there's something else driving it. The problem with the RP2350 is that (under some circumstances) there's a current leakage between the pin and the voltage rail, so when a pin is configured as input with a pull-down resistor, the voltage will not go down to the low level it needs to read a logical low as expected: it will be at around 2.2V, which is in the "undefined" region.
- axoltl 2y ago
  You are, of course, 100% correct. In my haste to explain the 'sourcing' behaviour of the errata I accidentally jumped to the sinking verbiage. Input pins are "pressure gauges", not "flow meters".

crest 2y ago

Simplifed when used as digital input a GPIO pin has three voltage ranges: 0V up to the logical low threshold, VCC down to the logical high threshold, and an and undefined range in the middle. The RP2350 burns a lot of power while in this middle range when any proper MCU shouldn't, because its quite common for this to happen. The "cure" is almost as bad as the problem because to stay out of this range you have to apply a strong external pull-up/pull-down resistor. This too wastes power, just not quite as much.