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Current sourcing and sinking of AT89C51CC03

Stef

Jan 19, 2017
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Dear

I am trying to find out how much an I/O pin of the MCU can source our sink.

I need to pull 4 enable pins of an ic high with 1mA with port 2 of the AT89C51CC03 that is powered on 3,3V (and voltage need to stay above 2V). But I am not sure how much the µC can source.

I was looking the the diagram below (page 168 -169 of datasheet) but I think I don't understand it correct.
I'm thinking that VOH means the following: If VCC is 3,3V and I make the pin high. Now I load the pin to the ground that there is a current of 60µA. The voltage on this pin now only will be 3,3-1,5 = 1,8V. Am I correct or where I'm I wrong?
I think I'm wrong because it has an internally pull up what I thought that was able to give much more current + 60µA is really not a lot. I was guessing more a value around 2 - 4 mA.

I hope somebody can explain my why I'm wrong and help me to understand :)
Kind regards
Stef




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Harald Kapp

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The voltage on this pin now only will be 3,3-1,5 = 1,8V. Am I correct or where I'm I wrong?
This exactly what the datasheet states. The datasheet gives this as a worst case parameter (Min.) but you should not rely on the actual part being able to deliver more. It may or may not.

I was guessing more a value around 2 - 4 mA.
Use port 0 then, It will deliver 2.4 V @ 3.2 mA or use an external driver/buffer chip.
 

Pyramid

Jan 17, 2017
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You could also use external pull ups on the port pins that can provide more drive current when high. If you look at the port construction on P1-P4, they have a FET to ground that can sink several mA and create a hard low when the port is written with a logic 0. They have a FET to Vcc to provide a hard high when port is written logic 1, but it is only active for a short period of time to improve the rise time when driving capacitive loads. This FET is switched off and a low current FET provides the weak pull up in a static condition when the port is written logic 1. This is the reduced current sink capability you are seeing in the data sheet. Adding an external pull up can increase this drive level, but make sure the port current is not exceeded when the port is written logic 0.
 
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Stef

Jan 19, 2017
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Thank you both for the usefull response.
So much better than a yes or no!

I gonne use the externaly pull ups. Longer rise times are no problem for the design.
 
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