Re: [avr-chat] STK500 LEDs

[John Day]

At 04:17 PM 10/17/2007, Joerg Wunsch wrote:
> "Graham Davies" <address@hidden> wrote:
>
> >> This is the standard way for driving LEDs, the micro
> >> can sink more current than it can source ...
>
> > This isn't really true.
>
> It used to be true by the time the STK500 has been designed.  All the
> early AVRs also worked that way (usually being able to sink about
> twice the current they could source).

And even many of the modern ones are better sinkers than sourcers.

The lower or pull-down device is likely to be an N-channel device 
which will typically have a lower drain to source on resistance than 
the upper P-channel device.

At 5V some AVRs are specified to source 3mA with Voh = 4.3V (so a 
0.7V drop) whereas on the same pin when sinking Voh = 0.6V at 20mA - 
thus lower package dissipation when sinking.

John

>   Also keep in mind that the
> STK500 was even intended for some MCS51 controllers (AT89S51 IIRC),
> and the MCS51 in general can *only* sink current on its outputs, not
> source.  They don't have a data direction register but only pullup
> resistors and a pull-down output driver transistor.  (At least, that's
> what I've read about them.  I never really used MCS51 myself.)
>
> --
> cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL
>
> http://www.sax.de/~joerg/                        NIC: JW11-RIPE
> Never trust an operating system you don't have sources for. ;-)
>
>
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