They can obviously take some abuse - the evidence is all around us.There's another limiting factor which is that the voltage output drops with increasing current.Look at VOH in the datasheet. With 6V Vcc and 7.8mA draw, the voltage drops to 5.2V.It doesn't specify values for higher currents than 7.8mA but with Vcc=5V and 20mA draw it will probably drop down below 3V.At 3V a Blue LED won't allow 20mA to pass. If you connect a blue LED to a pin you'll never reach 20mA because there won't be enough volts to push that much current through the LED. It will reach equilibrium at a value less than 20mA.The same sort of thing might happen to the whole chip, that's why they survive. As an experiment you could set it up to draw 20mA on each pin then turn the pins on one at a time and measure the current going into the chip (and on each pin). Draw a graph of the result and post it.Just because the chip survives doesn't mean it's good engineering. The LEDs will get dimmer, the chip could heat up...it might even damage it in the long term.
So you are proposing sourcing 70mA with 4 high output pins to Gnd outside of the chip,and sinking 70mA with 4 low output pins from Vcc outside of the chip?That would probablyt be okay, chip internal control currents are negligible. Got a feeling the chip would get warm. 70mA comes in VCC pins, goes the Gnd else where. 70mA from elsewhere comes in and goes out the Gnd pin.
Quote from: Nick Gammon on Nov 18, 2013, 10:21 amQuote from: fungus on Nov 18, 2013, 10:00 amYep. There's no confusion over it or "different answers". The specification says 70mA per chip.Per pin, in what I linked.The pins you're referring to are "Vcc" and "GND", not I/O pins.
Quote from: fungus on Nov 18, 2013, 10:00 amYep. There's no confusion over it or "different answers". The specification says 70mA per chip.Per pin, in what I linked.
Yep. There's no confusion over it or "different answers". The specification says 70mA per chip.
+/- 70 refers to VCC or GND pin, so I'd read that as 70mA in on VCC pin (70), and 70mA out on Gnd pin (-70).
+/- 70 refers to VCC or GND pin, so I'd read that as 70mA in on VCC pin (70), and 70mA out on Gnd pin (-70).What kind of voltages do you see across the LEDs & current limit resistors, with a 'scope?
0.6 / 150 = 0.004 = 4 mA
(3.3 - 2) / 150 = 0.0087 = 8.7 mA
Backwards? So only 4 on at time, but switched quickly? Sure, that could work.