Microcontroller GPIO

[AbhinavRajan]

How much Voltage will be present at the GPIO pin for a Microcontroller which has a Vcc(Input Supply voltage) of 3.3V ?
Suppose I need to connect an LED at this GPIO what are the factors or calculations that I have to consider ?

And, How much voltage should I apply to an LED to turn it on? What should be its supply voltage?

 

[Easyrider83]

Digital output is just a pair of transistors between ground and vcc. Basicaly, if top one is opened, the voltage on output equal to vcc.
LED should be switched on by appling the constant current, not voltage.

 

[KlausST]

Hi,

To both your questions there is only one valid answer:
--> look into the datasheets

There is no general answer, not for LEDs, nor for microcontrollers. Each device has it's own specifications.

Independet: engineer or hobbyist, experienced or not, we all have to read the datasheets.

You need to know the manufacturer and the exact device name, then go the manufacturer's internet site and download the datasheet.

The values for voltage and current are either given as values, formulas or charts. It will be there.

If you have problems in finding a solution, then post links to the datasheets (manufacturer) and explain what you found out, what's your calculations....then we will assist you to solve the problem.

Klaus

 

[KamalKumar]

Just check the your microcontroller output sourcing and sinking current in the datasheet. If you want to connect LED's cathode to ground and anode to the (through a resistor) output, you must look at sourcing capability of the pin. If you want to connect Led anode to vcc 3.3V and cathode to the output via a resistor, see the maximum sinking current possible. Now to choose LED current- smaller3mm leds light up reasonably bright at 5mA. When the current passes through the led there will be a voltage drop of roughly 1.2V across it. The series resistor value will be then 3.3-1,2/5=0.42K. Use nearest resister 470ohms.All this is assuming, your source/sink current is >>5mA.
Older C51s have poor source currents.