[TriEmbed] TriEmbed Digest, Vol 31, Issue 5
triembed at undecidedgames.net
Thu Dec 3 09:36:39 CST 2015
On 12/2/2015 11:33 AM, Chip McClelland via TriEmbed wrote:
> Brian, Mkaes perfect sense in steady state but could there be transient
> differences between the board's Vcc rail and the internal voltage inside
> the Arduino? Is this something I need to worry about?
Shane also replied to this, but I thought I would expand on it a little.
Take-home message: Vout physically cannot exceed Vin on a
microprocessor. Not "must not" but "cannot," i.e. it is impossible due
to conservation of energy (a law of physics).
Application: If Vcc sags, Vout from the digital pin will sag right
along with it.
No silicon junction has a zero "on" voltage. Even if the micro has
highly efficient internal drive circuitry that connects its digital
output pin as directly to Vcc as possible for a logic "one", it's still
flowing through at least one silicon junction and incurring a voltage
drop across that junction. It may be a drop on the order of a few tens
or hundreds of millivolts, but the drop is there. The output pin's
potential will *always* be lower than Vcc. No current at all will flow
through a driven-high pin connected directly to Vcc, because the drive
transistor won't be forward-biased.
The only way for an output pin to exceed Vcc would be through some
catastrophic internal event involving catastrophic external conditions,
and the pin-to-Vcc connection would be the least of your worries!
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