how to calculate Pull down resistor for 8051

[v.s.n.kumar]

Hi,
i want to connect pull down resistor to 8051 port1 pin. because i have selected that pin as input. & i gave the output from transistor bc547 collector to that pin. but the pin is always high even the transistor is acting as closed switch or open switch. so i decided to connect the pull down resistor. but i dont know what's the value. i didn't find in data sheet. so can anybody tell. i have already connected 330 ohms. i.e one terminal of resistor to ground & other to pin. but the pin value is high. it didn't change to logic low. so plz can anybody help me.

 

[IanP]

Hi,
i want to connect pull down resistor to 8051 port1 pin. because i have selected that pin as input. & i gave the output from transistor bc547 collector to that pin. but the pin is always high even the transistor is acting as closed switch or open switch. so i decided to connect the pull down resistor. but i dont know what's the value. i didn't find in data sheet. so can anybody tell. i have already connected 330 ohms. i.e one terminal of resistor to ground & other to pin. but the pin value is high. it didn't change to logic low. so plz can anybody help me.

I don’t think you work with the “old” 8051, instead your derivative is more likely to be AT89S52, or something similar. They don’t have internal pull-up resistors, as resistors take up a lot of area in CMOS.
What you can find there will be a small current source made out of MOSFET(s) and limited to roughly 50uA. If you can’t pull it down to 0V with a 330ohm resistor – the pins internals can be damaged.
:wink:
IanP