The AT89S8052's pins are configured as bi-directional input-outputs. The way I understand that they work is by weakly pulling up an internal drain terminal. They're capable of delivering very little current, they were designed to drain current instead. The datasheet does not say what resistor value is internally being used to pull the pin up.
Anyway, I need to detect a signal level change on those bi-directional pins arriving from a sensor that has a PNP output (its high output level is 5V, so we're safe as far as voltage is concerned). My plan is to pull the MCU's pins down with a resistor, so that when the sensor is activated it will bring the voltage back up.
I've tried to represent what I think is happening inside the MCU, and what I plan to do to interface it to the PNP sensor. I hope my depiction is more or less accurate.
My question is, what's the maximum value allowed for those pull down resistors so that the MCU will detect a low signal at its pin? I'm under the impression that the higher, the better, so as to draw as little current as possible from the PNP sensor when it goes high.
@Papabravo I have a hunch that you might be able to help me out with this one.
Anyway, I need to detect a signal level change on those bi-directional pins arriving from a sensor that has a PNP output (its high output level is 5V, so we're safe as far as voltage is concerned). My plan is to pull the MCU's pins down with a resistor, so that when the sensor is activated it will bring the voltage back up.
I've tried to represent what I think is happening inside the MCU, and what I plan to do to interface it to the PNP sensor. I hope my depiction is more or less accurate.
My question is, what's the maximum value allowed for those pull down resistors so that the MCU will detect a low signal at its pin? I'm under the impression that the higher, the better, so as to draw as little current as possible from the PNP sensor when it goes high.
@Papabravo I have a hunch that you might be able to help me out with this one.