Electronic – Help identifying a potentiometer/resistor like symbol in the I/O stage of a microcontroller

identificationresistorsschematicssymbol [~]

I'm having difficulties identifying a schematic symbol. I spotted this while going through the STM32L series reference manual on page 172 located HERE.

Here is the symbol in question, and the symbol as it appears in the document:

The symbol

Part of the schematic

It's obvious that it's some sort of electronically switchable resistor, I'm just uncertain what the proper name for it is. I thought it was a Voltage Controlled Resistor however I've generally seen those in schematics as the actual FET, as opposed to a symbol like this. My searches haven't turned up anything.

I know they're the pullups/pulldowns for the port, I just have never seen that symbol before.

Does anyone recognize this symbol or know what the proper name for it is?


Best Answer

They're probably intended to represent small transistors operating in the linear region when used as (configurable) pull-ups or pull-downs. True resistors of such high value are expensive (large acreage required) to make in many VLSI digital processes so transistors are used. See this reference.

Because the effective resistance of such a pull-up/pull-down is not constant and because it can be switched, the symbol represents it more accurately than a simple MOSFET in series with a resistor.

Keep in mind that commercial I/O circuitry that meets ESD and other requirements is generally fairly complex and proprietary and you can be sure that whatever you are seeing is a simplified version. See, for example, this student thesis.