IO current sinking

[PW_CTL]

Hi guys,

I am reading the datasheet of the MC9SH8QG4 and I see next data:

Maximum total IOH for all port pins | IOHT | — 60 mA

Maximum total IOL for all port pins IOLT — 60 mA

I want to power about 8 leds, and each one has a current consumptiom about 50mA. Can I power the leds directly with the microcontroller or shall I need to use transistors? I am not sure if the "maximum total IOL/H" is for one pin or all the pins of the microcontroller.

[peg · Accepted solution]

Hello,

This is the sum of all the pins sinking or sourcing at any one time. You will need some form of external driver.

[layman]

Hello,

I have a similar confusion with MC9S08SG8. I want to power a 10ma led directly with the controller. All other i/o pins are not sourcing/sinking any significant current. The datasheet does not seem to set any limit on individual pin but there is an overall limit of (Max total IOH for all ports OHT/OLT = –/+100 mA.Can any one please confirm that its safe to sink 10mA thru a single pin of MC9S08SG8 ?

Thanks

Pramod

[bigmac]

Hello Pramod,

Sinking 10 mA at a single pin is quite OK (this thread originally refered to sinking 50 mA). However, you will need a suitable series connected resistor to limit the LED current.

Actually, for this current level to be reliably achieved, a Vdd of 5 volts will be necessary, in conjunction with high drive current.  Even so, the LED current may vary from unit-to-unit because the the pin output voltage may be up to 1.5 volts with this level of sink current (think of this as a variable output resistance up to 150 ohms maximum). 

Regards,

Mac

Message Edited by bigmac on 2009-10-15 06:28 PM

[layman]

Mac, Thanks for the detailed reply. 

The orignal post was about MC9SH8QG4 while I am using MC9S08SG8.

Being a newbie to freescale, just wanted to be sure before I send the gerber out.

From the datasheet, I can see that for ILoad = 10 mA. Output low voltage (Max)= 0.8. I understand that this means that my pin voltage can be anything between 0 and 0.8 instead of  being exactly equal to 0.

Is this the phenomenon that you are refering to when you say "variable output resistance up to 150 ohms " ? 

Am i correct in saying that the Variable o/p resistance can be anything between 0 at 80 ohm ?

Regards

Pramod

[bigmac]

Hello Pramod,

I did actually check a copy of the datasheet for the 'SG8 device, and the specified maximum voltage was 1.5 volts at 10mA.  This was a preliminary version, so perhaps the specification has changed for the latest datasheet.  Based on the figure you quoted, the maximum output resistance would indeed be 80 ohms. 

The minimum resistance would never be zero, but I guess might possibly reduce by a factor of two or three for some devices.

Regards,

Mac 

[layman]

Thanks a lot Mac.

[PW_CTL]

Thanks. Then, I´ll put some little transistors.