BX-24 I/O Pins

Started by Mario June 6, 2005
Good Morning,

I have an application where I am using the ADC of the BX-24 to read
some resistance values of a wheatstone bridge sensor. I am using
relays, constant current source, and active circuits to read and scale
a voltage to correspond to a resistance. The problem that I am running
into is that when the relays are not engaged my analog circuit is
outputting about 11 volts on an op-amp. This op-amp is directly
interfaced to the BX-24 IO pin. The Pin is in High-Z mode at this
time, when a resistance reading is taken the voltage is between 0-5
volts. I then decided to add a 6 Volt clamping zener on the input of
the op-amp to clamp this voltage. However the zener does introduce
some non-linearities, but I found a specific zener that did a good job
with non-linearities I could live with. My question is how high of a
voltage can the BX-24 pins take in High-Z mode?


Mario wrote:

> Good Morning,
>
> I have an application where I am using the ADC of the BX-24 to read
> some resistance values of a wheatstone bridge sensor. I am using
> relays, constant current source, and active circuits to read and scale
> a voltage to correspond to a resistance. The problem that I am running
> into is that when the relays are not engaged my analog circuit is
> outputting about 11 volts on an op-amp. This op-amp is directly
> interfaced to the BX-24 IO pin. The Pin is in High-Z mode at this
> time, when a resistance reading is taken the voltage is between 0-5
> volts. I then decided to add a 6 Volt clamping zener on the input of
> the op-amp to clamp this voltage. However the zener does introduce
> some non-linearities, but I found a specific zener that did a good job
> with non-linearities I could live with. My question is how high of a
> voltage can the BX-24 pins take in High-Z mode?
>
According to the Atmel documentation for the AT90S8535, max Vcc is 6V
and max input voltage is Vcc+0.5. I think this means that even in High-Z
mode the max voltage is 5.5V.

I think you need an alternative like a voltage divider. you could gate
in the op-amp output only when the BX-24 needs it but then you need some
kind of safety so that you don't inadvertently send the 11V to the BX-24
through programming error or otherwise i.e. the hardware backs up the
software.

Mike



--- In basicx@basi..., "Mario" <domovoi22@y...> wrote:
> Good Morning,
>
> I have an application where I am using the ADC of the BX-24 to read
> some resistance values of a wheatstone bridge sensor. I am using
> relays, constant current source, and active circuits to read and scale
> a voltage to correspond to a resistance. The problem that I am running
> into is that when the relays are not engaged my analog circuit is
> outputting about 11 volts on an op-amp. This op-amp is directly
> interfaced to the BX-24 IO pin. The Pin is in High-Z mode at this
> time, when a resistance reading is taken the voltage is between 0-5
> volts. I then decided to add a 6 Volt clamping zener on the input of
> the op-amp to clamp this voltage. However the zener does introduce
> some non-linearities, but I found a specific zener that did a good job
> with non-linearities I could live with. My question is how high of a
> voltage can the BX-24 pins take in High-Z mode?

Consider a 22K series resistor between the op amp output and the BX24
input. The theory here is that the electro static clipping diode (in
the BX24) will turn on above 5.1 or below -0.7 and the voltage
difference, in your case 7 VDC, is across this series protection R.
I don't think it necessary, but better safe than sorry, to locate the
6 VDC clamping diode you have right on the input of the BX24. But, as
I say, I think this is redundant with the internal clipping diode.

This series 22K is used all the time to interface wit RS232 devices.

P H Anderson, http://www.phanderson.com/basicx/


I may well be wrong here but the standard BX-24 and it's Atmel chip
does not have clamp diodes? I know the BX-24/P with the ATmega does
as does PIC chips.

- Tom

--- In basicx@basi..., "pha555" <pha@p...> wrote:
> --- In basicx@basi..., "Mario" <domovoi22@y...> wrote:
> > Good Morning,
> >
> > I have an application where I am using the ADC of the BX-24 to
read
> > some resistance values of a wheatstone bridge sensor. I am using
> > relays, constant current source, and active circuits to read and
scale
> > a voltage to correspond to a resistance. The problem that I am
running
> > into is that when the relays are not engaged my analog circuit is
> > outputting about 11 volts on an op-amp. This op-amp is directly
> > interfaced to the BX-24 IO pin. The Pin is in High-Z mode at
this
> > time, when a resistance reading is taken the voltage is between 0-
5
> > volts. I then decided to add a 6 Volt clamping zener on the
input of
> > the op-amp to clamp this voltage. However the zener does
introduce
> > some non-linearities, but I found a specific zener that did a
good job
> > with non-linearities I could live with. My question is how high
of a
> > voltage can the BX-24 pins take in High-Z mode?
>
> Consider a 22K series resistor between the op amp output and the
BX24
> input. The theory here is that the electro static clipping diode
(in
> the BX24) will turn on above 5.1 or below -0.7 and the voltage
> difference, in your case 7 VDC, is across this series protection
R.
> I don't think it necessary, but better safe than sorry, to locate
the
> 6 VDC clamping diode you have right on the input of the BX24. But,
as
> I say, I think this is redundant with the internal clipping diode.
>
> This series 22K is used all the time to interface wit RS232 devices.
>
> P H Anderson, http://www.phanderson.com/basicx/