Speeduino is now on Github Sponsors (Rather than Patreon): https://github.com/sponsors/noisymime
Any general discussion around the firmware, what is does, how it does it etc.
#46543
JHolland wrote:
Wed Nov 18, 2020 2:53 pm
The ADCs in the STM32 are far superior to the single ADC in the Mega, you're just downgrading and adding complexity for no real reason.
There should be at least on paper. But in practise I have been strugling to get even noise free 10bit readings from those.
#46554
pazi88 wrote:
Wed Nov 18, 2020 4:57 pm
JHolland wrote:
Wed Nov 18, 2020 2:53 pm
The ADCs in the STM32 are far superior to the single ADC in the Mega, you're just downgrading and adding complexity for no real reason.
There should be at least on paper. But in practise I have been strugling to get even noise free 10bit readings from those.
What I was mostly refering to was more ADCs, the ability to perform auto-conversion on mulitple channels, faster performance, advanced triggering options, DMA.....

Have you got your anti-aliasing caps correct? - they are completely wrong on every Speeduino board I've seen. Also the cap should be very close to the micro pin, ideally with no connections or vias. Seperate analogue reference voltage, properly de-coupled? good PCB layout? The STM32 is a lower voltage so it will tend to be worse, OEMs use 5V processors for powetrain, however, most issues aren't the processor, at around 3mV per bit it needs good design. On the plus side the STM32 is fast enough to be able to oversample which allows the use of digital filtering.
#46555
JHolland wrote:
Thu Nov 19, 2020 9:19 am

Have you got your anti-aliasing caps correct? - they are completely wrong on every Speeduino board I've seen. Also the cap should be very close to the micro pin, ideally with no connections or vias. Seperate analogue reference voltage, properly de-coupled? good PCB layout? The STM32 is a lower voltage so it will tend to be worse, OEMs use 5V processors for powetrain, however, most issues aren't the processor, at around 3mV per bit it needs good design. On the plus side the STM32 is fast enough to be able to oversample which allows the use of digital filtering.
Do you have example of what is correct, tested and verified to lead noise free ADC inputs? I just linked my design on last page. For latest one I copied how rusefi does the decoupling but caps are closer to mcu than in those designs. Don't know yet how well that will work. Analog voltage reference now comes from 5v using same voltage divider resistors as are used with analog inputs to tie in the analog vref to 5v that all sensors use = differences in 5v voltage do not affect analog readings.
#46571
pazi88 wrote:
Thu Nov 19, 2020 10:59 am
Do you have example of what is correct, tested and verified to lead noise free ADC inputs? I just linked my design on last page. For latest one I copied how rusefi does the decoupling but caps are closer to mcu than in those designs. Don't know yet how well that will work. Analog voltage reference now comes from 5v using same voltage divider resistors as are used with analog inputs to tie in the analog vref to 5v that all sensors use = differences in 5v voltage do not affect analog readings.
The resistor divider is a good idea but you have a lot of filtering so Vref may not follow the 5V line well. The problem with the Mega format is that the analog and digital returns follow the same path so you pick up noise on the 0V line. You are also subject to any noise that might occur from the shield. What else is your 5V driving? How much noise are you seeing?
If the anti-aliasing filters aren't well designed then you will get a lot of additional noise. I did a similar design with an SPC572 and added provision for the filter caps on the board to get them the 'right side' of the connector, it won't be much of an improvement though.
Noise is going to be a problem with a design like that and there isn't a simple solution, you only really need 8-bits so noise on the last two bits isn't really an issue and you can use the speed of the STM32 to oversample to reduce noise without reducing the bandwidth.
The idea of a seperate ADC could work well with a filtered 5V supply to the sensors and its reference it would decouple the inputs from the noise on the 3V3.

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