[arm-allstar] diode

[David McGough]

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
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