prolem in 8051 sinking current

[Arunmag]

i am using at89s52. i programed so all ports are 0 so i connected led in it..
inside controller 0 means it is connected to ground directly.
but when i power the mcu i get only dim led light..
is there a cureent sink even in ground.
is there any way to customise our our micro controller ie/ no of ports, interupts etc

 

[KerimF]

it is connected to ground directly

It is connected to ground via an open collector NPN transistor (the internal pull-up resistance is relatively high) with a limited internal Ib hence external Ic in order for its Vce be low (close to ground).
When I connect an LED to a port pin, I add a resistor in series to limit the LED current be not more than 10mA (less if there are many LEDs). For a higher current I use a buffer NPN, PNP, MOSFET or IC.

 

[Arunmag]

i have to use 2 port completly for led. using external transistr makes it complex..
i am running the led so fast via bit changing.. will that cause it..

how to get a custom mcu unit for our product etc

 

[sherazi]

can u share your circuit ?? how are you connecting the LED?

 

[yuvko]

how fast are you changing the bit controling the LED? maybe it is too fast for the eye, try a slower change.

 

[KerimF]

Sorry I thought the current is rather constant (DC). It is normal if the current is pulsed at a relaively high frequency (say above 100Hz), the LED brightness could be controlled by varying the duty cycle.
I am sorry I couldn't see where your problem is. Are your LEDs of power type and not the small standard ones?

 

[srizbf]

i have to use 2 port completly for led. using external transistr makes it complex..
i am running the led so fast via bit changing.. will that cause it..

how to get a custom mcu unit for our product etc

whether one LED or many , 89s52 cannot drive LEDs directly from port. you have to use a buffer for driving LED(bjt or ic buffer).

 

[KerimF]

whether one LED or many , 89s52 cannot drive LEDs directly from port

Would you please explain why?

 

[srizbf]

Would you please explain why?

the source and sinking current capability of 8051 ports makes that limitation.

 

[KerimF]

the source and sinking current capability of 8051 ports makes that limitation.

I agree with you. The port cannot source an acceptable LED current (via the internal high pull-up resistance) if not at all (when no pull-up).
But I always use the port without any buffer to light LEDs (mainly for signalling) via limiting resistors (pin low). To save power, I let the LED current around 4mA (sometimes less) though up to 20mA is possible. But if there are more than one LED, the total current should not exceed a limit; like 80mA for AT89C2051.
I am talking about AT89C2051, AT89C51/52, AT89S51/52, AT89S8252/53 and SST89E58RDA which I have. I am not sure about other ICs