Electronics > Microcontrollers

Why do internal pullup resistor's have a range ?

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MathWizard:
For example in the ATtiny13A datasheet, the internal pullup resistor is listed as from 20 to 80k, and no typical value.

So why such a wide range, I'm sure there's fixed resistor's with under 5% tolerance in this MCU. So why not use one for the PU ??, with a known value ??

Or is it just the V/I, taking into account the different voltage losses on the MOSFET the sits on top the Rpu to Vcc ?

Siwastaja:

--- Quote from: MathWizard on October 28, 2024, 08:33:41 am ---I'm sure there's fixed resistor's with under 5% tolerance in this MCU

--- End quote ---

Accurate resistors are possible to manufacture on an IC, but they are more expensive. For pull-up purposes exact value does not matter at all so they use a cheaper process.

MathWizard:
So it really is just a high tolerance resistor ?

Ok so don't trust them in voltage divider's then too. I'm just finishing baremetal coding my own I2C program, and yeah I shouldn't trust the PU state, with the series resistors I have going to the EEPROM I'm testing with. I have 4.7k PU's on the BUS lines too.

ArdWar:

--- Quote from: MathWizard on October 28, 2024, 08:33:41 am ---I'm sure there's fixed resistor's with under 5% tolerance in this MCU.

--- End quote ---
Are you sure?

Making accurate resistor in silicon is not trivial at all. At least not in large volume low cost friendly process. That's why ICs are overwhelmingly designed in such a way where their functions and performances are tied to value ratio instead of absolute value.

In this particular case, even if the absolute accuracy between units is rather lax you're more likely to find all 100 pull up resistor inside a single MCU to be within say 2% of each other.

Chalcogenide:
For such a large tolerance I would assume that they are not using real resistors, but transistors. Some MCUs that actually use resistors (i.e. most STM32) write it into the datasheet and have a narrower range (i.e. 25k to 55k, typ 40k). In fact, having a tight tolerance resistor (5% is already a problem) on a silicon die almost always require trimming that is not justified for a simple pull-up functionality where the exact value is just not needed.

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