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My understanding is that MCUs (especially 32 bit ones) typically operate at 3.3 V. I have a 24/12 V, 32 bit automotive ECU (Bosch Rexroth Series 40). I've provided them as an example, but I'm asking generally about all automotive ECUs. Their datasheet includes a block diagram as follows:

Block diagram of the Bosch Rexroth Series 40

This leads to my questions:

  1. Does the "Voltage supplies and watchdog" block step-down the supply voltage to 3.3 V for the μC block?
  2. Do the ECU input conditioning blocks step down the voltage before feeding into a 3.3 V ADC? I've included another diagram to visualize what I mean. Conversely, are outputs stepped up from 0-3.3 V to the appropriate 0-12/24 V using amplifiers?

An analog signal being fed into a DC/DC converter or resistor divider, which leads into an ADC, which transmits serial communication to the μC

Edit: Thanks for the comments. For context, I'm specifically asking about the internals of the ECU (MCU block and peripherals,,) not about external sensors and their supplies. Below is an application that I'm using this controller for. It monitors the supply voltage for undervoltage and uses the supply voltage for calibrating parameters in firmware. As you can see, it does not rely on the sensor supply outputs. It is purely reading the supply voltage.

The ECU is supplied by 12-24 V while also reading the supply using an analog input pin

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  • \$\begingroup\$ Why do you think cars use 12V? I have driven cars which have a 48V as it's battery, and others with 415V. You're used to a 12V lead acid battery which is used to power the lights etc. That is because that's a convient votlage for those devices. This voltage gets stepped down to something useful for each device. For example in the radio it gets stepped down to 1.8V or even 1V for the components in it, the same goes for the ECU. Like your computer at home, it gets 110V/220V AC in, it gets stepped down to a useful voltage inside the computer. \$\endgroup\$
    – Puffafish
    Commented Dec 5, 2023 at 17:15
  • \$\begingroup\$ More or less, yes. Did you want a more specific answer to any of these questions? \$\endgroup\$
    – Tim Williams
    Commented Dec 5, 2023 at 17:15
  • \$\begingroup\$ Here's a better link for the datasheet... According to this datasheet, there are four VSS pins that can be used for referencing: VSS1 to VSS4, stands for "Voltage Sensor Supply", contrary to normal reference nomenclature, VSS1 to 3 says it outputs 5V and 4 outputs 10V. You'd need a separate PMIC to step down to 3.3V. \$\endgroup\$
    – user319168
    Commented Dec 5, 2023 at 17:16
  • \$\begingroup\$ Some questions which may help you: electronics.stackexchange.com/questions/6403/… and electronics.stackexchange.com/questions/665524/… \$\endgroup\$
    – Puffafish
    Commented Dec 5, 2023 at 17:18
  • \$\begingroup\$ We can't know how this specific device or similar devices work. It might use 3.3V or some other supply. Sometimes inputs are externally divided before ADC, sometimes you feed higher voltages directly to ADC, and it depends on ADC. Outputs might not be stepped up with amplifiers, just driven with transistors through level shifter - but whether you count that as an amplifier is another thing. Unless you find schematics or open the ECU and do measurements and reverse-engineering, these questions cannot be answered. \$\endgroup\$
    – Justme
    Commented Dec 5, 2023 at 17:43

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It’s impossible to make a blanket statement about what voltages the MCU will actually use - you need to consult the MCU datasheet.

That said, 5V and/or 3.3V are common for logic I/O voltages, it's highly likely that the MCU has at least this voltage rail. The MCU core voltage for a newer, more advanced MCU could be lower, such as 1.2V or even lower than that. This is compatible with more advanced process nodes.

As for what voltage range the MCU’s ADC supports, again you’d need to consult the data sheet. In any event the sensed voltages will need to be scaled to what the ADC can accept.

If you are looking to use a power rail for the voltage reference, it would seem prudent to instead provide a dedicated reference that is stable and also qualified for automotive.

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