[arm-allstar] diode

[Doug Crompton]

I will more simply translate that.

The diode allows a signal on the radio side to pull the FOB COS line low
and allows it to go high solely based on the FOB internal pullup. At the
same time it stops any positive voltage (possibly above the FOBS limits)
from entering the FOB.

The CM1xx while running on 3.3 volts its pins are 5V tolerant -

Dvinout Voltage on any Digital Input or Output Pin  –0.3 to +5.5V
Avinout Voltage on any Analog Input or Output Pin  -0.3 to +5.5V

So if you had a 5V or less driving source you could safely eliminated the
diode. The diode can be a problem in some cases where its forward voltage
drop is too high and it does not bring down the input line to the FOB
properly. This is why a Schottky (low forward drop) diode is specified.
This is especially true when the radio does not bring the line down to very
close to 0 volts adding to the problem. That being said make sure the input
voltage from the radio never exceeds the 5 volt TTL level before
eliminating the diode.


*73 Doug*

*WA3DSP*

*http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*


On Thu, Aug 3, 2017 at 5:24 PM, "David McGough via arm-allstar" <
arm-allstar at hamvoip.org> wrote:

>
>
> Hi Harry,
>
> The CM1xy sound chips are 3.3V devices. The CM1xy GPIO pins include an
> internal pull-up resistor, attached to the 3.3V rail. So, if the pin is
> left unattached to anything, it'll "float" to a 3.3V level, logic high.
>
> With the diode in the line, anode end attached to the sound chip, this
> provides a simple way to allow the COS signal voltage range to go all the
> way from 0V to at least 20V or higher, limited only by the diode switching
> time and reverse breakdown voltage.
>
> ...So, for example, lets say the COS positive voltage is 10V. Since the
> diode anode = 3.3V and cathode = 10V, the diode is reverse-biased and
> turned off...This allows the CM1xy GPIO pin to "float" at 3.3V, a logic
> high level.
>
> ...Once COS goes to the opposite state, close to 0V, the diode anode =
> 3.3V and cathode = 0V. The diode is now forward-biased and turns on,
> pulling the GPIO voltage down to a low level, approximately 0V + the diode
> forward voltage drop. For a small-signal Schottky diode, the forward
> voltage drop will be about 0.3V....The GPIO pin is now a logic low.
>
> Does this make sense?
>
> 73, David KB4FXC
>
>
>
> On Thu, 3 Aug 2017, "Harry Romano via arm-allstar" wrote:
>
> > What function does the diode that is inline with the cos line do. I use
> a TTL 5 volts interface so do most newer radios how is it useful???
> >
> > 73
> >
> > Harry
> > kc4rpp
> > _______________________________________________
> >
> > arm-allstar mailing list
> > arm-allstar at hamvoip.org
> > http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
> >
> > Visit the BBB and RPi2/3 web page - http://hamvoip.org
> >
>
> _______________________________________________
>
> arm-allstar mailing list
> arm-allstar at hamvoip.org
> http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
>
> Visit the BBB and RPi2/3 web page - http://hamvoip.org
>
>