Saturday, 1 August 2015

atmel - Is it OK to attach an LED directly to a 5V Attiny?


From the datasheet, I though the AT90S1200 had current-limited pins and when running at 5V would sink the proper amount of current through a green LED attached to + (0 turns on the LED, 1 turns it off) without any external resistors. Unfortunately when I attached 8 of them the IC burned out after a few minutes. I also have a dodgy solderless breadboard and may have otherwise shorted out the part.


What did I misunderstand about that datasheet?



Answer



From the data sheet


DC Current per I/O Pin ............................................... 40.0 mA


DC Current VCC and GND Pins................................ 200.0 mA


Now each led will be over driven at 40mA (as there is no current limiting resistor) - enough to shorten the life of the led but otherwise not too bad. However running 8 leds will draw a total of 320mA from the microcontroller - together with any current it itself requires to run (depends on clock speed). This is enough to greatly exceed the 200mA total allowed, hence your burned out chip.


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