hoping i can power a couple of LED displays (30mA total draw) from the 3.3v out but certainly dont want to frazzle my mbed (having too much fun with the thing). question then is how much current will the 3v3 output source safely ?? tnx...

garyb

A Digital output pin won't drive more than a few mA. The Vout (pin 40) max current depends on where the power is coming from. If you use USB as a powersource the max current is maybe 200 mA before the PCs USB currentlimit kicks in. The max current could be a bit more when you have an external powersource on Vin. The 3V3 regulator on mbed should go upto 800mA I believe. I have used Vout in combination with a USB powersource to drive a large number of LEDs in multiplex mode. See http://mbed.org/users/wim/notebook/starburst-led-display/ Multiplexing will save a lot of power (and pins) without losing much brightness. I assume your LEDs are meant to provide information rather than illumination :)

Thanks Wim. Yes, i would like to power an 8 digit (x7 segment) display via an AS1107 (multiplexed). Sounds like I can do that without difficulty then. I think this set up allows me to check my code and hardware anyway.

I multiplex my LED display at 75Hz per digit using 3V3 and a 56 Ohm series R for each segment. That would be 3mA continuous current per LED. The effective current is reduced by 4 in case of 4 digits. Start with low currents and work your way up. Check the LD1117 for overheating using your finger (or tongue if you prefer that...)

xD, I currently imagine how people lick the chips on the mbed to feel their temperature.

Try a much more display rate instead of 75Hz .. something like 1KHz. And also try to implement some kind on brightness control by modifying the time that one digit is selected from the total amount of time. This way you have a software control on the brightness and the power disipation is more appropiate to your needs.

A higher update rate as such wont make a difference: with 4 digits they will each still be on for 25% of the time. The rate of 75hz was selected as the minimal frequency needed to avoid visible flicker. Increasing the update rate and disabling the digit for a percentage of the updates will result in brightness control but at the cost of more complex code and higher cpu overhead. In the example schematic that I gave above a provision was made instead to use the mbed PWM output for brightness control.