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siemens / meta-iot2050

SIMATIC IOT2050 Isar/Debian Board Support Package
MIT License
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Different AI resolution in python and node-red #407

Closed bergmanu closed 1 year ago

bergmanu commented 1 year ago

Using the GPIO node-red node to read in an analog value from the I/O Module - either 0..10V or 0..20mA - results in a value from 0..4095 (12-bit resolution)

Using the same analog input with mraa in python the resolution is only 0..1023 (9-bit)

Where does this difference come from?

AsuraZeng commented 1 year ago

After my investigation, node red set the ADC bit to 12bit, but the default setting in mraa is 10bit. The current workaround is to set the bit of adc to 12 after initializing the ADC.

I have a PR to resolve this one. https://github.com/eclipse/mraa/pull/1103. it's under review currently.

jan-kiszka commented 1 year ago

As you correctly noted: That PR is a workaround, not the solution yet.

jan-kiszka commented 1 year ago

And: there is no issue. node-red-node simply raises the number of bits to what the hardware can do. That is done at https://github.com/node-red/node-red-nodes/blob/9bae313b722147b00480ce2e26ed0aab357b836b/hardware/intel/mraa-gpio-ain.js#L15. That is logically one level above what the Python (or also NodeJS) bindings does which only exposes the AIO abstraction. And that abstraction chose to start with 10 bits, hardware-independently.

IOW: Nothing to fix here or elsewhere, just to understand logic-wise.

jan-kiszka commented 1 year ago

In that light, setting the number of bits unconditionally in node-red-node could even be questioned from a hardware abstraction POV. It should rather become a tunable of that node.