Re: Microcontroller with 7V supply and I/O tolerance ?
From: Paul Keinanen (keinanen_at_sci.fi)
Date: 10/25/04
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Date: Mon, 25 Oct 2004 08:29:15 +0300
On 24 Oct 2004 05:17:17 -0700, mangled_us@yahoo.com (David) wrote:
>But still... a device with a higher supply and I/O voltage
>tolerance must surely have many applications ?
I do not see much need in a higher supply voltage, but higher I/O
voltages would be nice. I think that the largest problem is due to the
common use of multifunction pins, i.e. pins that can be programmed
both as inputs or outputs, which requires a lot of electronics on the
pin. Designing such pins for high I/O voltages would cause a lot of
reverse biasing problems.
However, if dedicated input on output pins are used, the outputs could
be simply open collector(drain) types, in which the external I/O
voltage could be quite high, provided that there are no real or
parasitic diodes from the output pin (collector/drain) to the Vdd. If
the pin can sink sufficient currents (>20 mA) LEDs and small relays
could be driven directly.
On the input side a high current protection diode would be required
between each input and Vdd and the input can be used as voltage input,
if the input voltage is always between 0 and Vdd or as a current
input, with an external series resistor, if the input voltage can
swing above Vdd. An extra resistor from input to ground may be needed
to move the "low" state threshold sufficiently low.
A CPU with a low internal power consumption can be driven with a
series resistor and a shunting zener from any voltage. The zener
should keep the Vdd below the maximum allowed Vdd and should be big
enough, so that it can also dissipate the total worst case input pin
current flowing trough the external series resistors through the input
protection diodes to Vdd and through the zener to ground
As such, I do not see a problem if the CPU core Vdd is quite low
(1.5-3.3V) but it sure would be nice to have (high voltage tolerant)
_current_ inputs and outputs (instead of voltage I/O).
Paul
- Previous message: Earl Bollinger: "Re: Microcontroller with 7V supply and I/O tolerance ?"
- In reply to: David: "Re: Microcontroller with 7V supply and I/O tolerance ?"
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