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?


  • 1
    \$\begingroup\$ uC's often have internal "active pullup" which is selected by register for some applications. Other IC's have have active pullup/down for apps like bi-directional buses with termination or bias to Vdd/2 when idle. I dont know of any other name than it's function ie. internal optional Pull-up or down R.... or internal switched active termination R's to Vdd/2 \$\endgroup\$ May 5, 2017 at 1:40
  • \$\begingroup\$ Yeah. I'm familiar with the function they perform, I just haven't seen that symbol before. Was wondering if there was something special about it or it ST just made up something. \$\endgroup\$ May 5, 2017 at 1:45
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    \$\begingroup\$ me neither, since it's an internal CPU schematic, we never use it on external schematics since it has no part or ref.des., Symbols evolve as they are invented. Next maybe a symbol for wireless 3D VR eyeball projector for glasses that replace all smart phones in the next decade. \$\endgroup\$ May 5, 2017 at 1:47

2 Answers 2


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.

  • \$\begingroup\$ Neat, those links are going to make some interesting reading. Thanks for the reply. \$\endgroup\$ May 5, 2017 at 23:41

These are just "selective pull-up and pull-down R's" for the I/O port, with no external common symbol name as they are virtual to we as users.

configured by software in several modes:

  • Input floating
  • Input pull-up
  • Input-pull-down
  • Analog Output open-drain with pull-up or pull-down capability
  • Output push-pull with pull-up or pull-down capability
  • Alternate function push-pull with pull-up or pull-down capability
  • Alternate function open-drain with pull-up or pull-down capability


AMD guys probably have an acronym for these parts like puppy R's or programmable PullUPDnRegister y's

  • PUPDRy[1:0]:
    • Port x configuration bits (y = 0..15)
  • These bits are written by software to configure the I/O pull-up or pull-down
    • 00: No pull-up, pull-down
    • 01: Pull-up
    • 10: Pull-down
    • 11: Reserved
  • \$\begingroup\$ I agree with that. Further down they talk about setting the ports speed with 4 options ranging from "Low Speed" to "Very High". Makes sense to have adjustable pulls to ensure an adequate slew rate. \$\endgroup\$ May 5, 2017 at 2:10
  • \$\begingroup\$ i'd just call em pup's and pud's \$\endgroup\$ May 5, 2017 at 2:18
  • \$\begingroup\$ @alphasierra Slew rate is separate from pull up/pull down; you don't have to set up a pull to control slew rate. The pull up/pull down resistor values are not configurable. \$\endgroup\$
    – user39382
    May 5, 2017 at 6:42
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    \$\begingroup\$ They specify a "light" load internally and no mention of values. But IF they have a speed option that changes the value , I agree a few orders of magnitude for high speed termination, but no values are given in my cursory read. \$\endgroup\$ May 5, 2017 at 13:07
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    \$\begingroup\$ Agreed. Looks like they are using a FET as a variable resistor. Lower value would result in faster rise times. NOTE: Symbols on chip schematics are often only functionally indicative of what they do. The actual device can range from what is shown to a whole circuit or even subsystem. \$\endgroup\$
    – Trevor_G
    May 5, 2017 at 13:52

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