Hi i'm trying to connect a TI-83 to an avr so i can communicate with different TI's via rfm12 from hoperf.
My doubts are now relative to the TI Link Port hardware because in here (http://zastava.student.utwente.nl/linkguide/hardware.html) and other sources it states Vcc -> 10k resistor -> diode -> A
I'm assuming that the transistor is off given i get 5.3V in point A and makes sense according to protocol. To test if this circuit was like this or not i tried using different values of resistance to the ground
Vp -> 10k resistor -> diode -> A -> My resistor -> ground (common to calc ofc)
Bu the readings I get are for instance:
MyR - V point A
9.89K - 3.489V
995 - .858V
4.94K - 2.587V
And from this values I am not able to verify that circuit, because it seems more like:
Vp -> 5219Ohm -> point A -> ...
i = (Vp - Vd - Vm) / Rin
For i=352uA (MyR=9.89k) with Rin=10K the drop on the diode would be -1.7V...
So maybe the 10K value was wrong so i used 2 measure i values for different MyR values and made Rin=Rin
Rin = (Vp-Vd-Vm)/i
i1=i2 <-> (Vp-Vd-Vm1)/i1 = (Vp-Vd-Vm2)/i2 wich gives me Vd=0.0194V => Rin=5175ohm (ouch)
Could it there be such a low drop diode?
Is anything wrong with my thinking here?
Appreciate any help, thanks! :)
As for the connection to the AVR for instance in the fastAvrlink http://zastava.student.utwente.nl/linkguide/cable.html the pins from the AVR go directly to point A. But the VA when high is higher than 5V, isn't this bad to the port? I know the pins can take for instance Vcc+0.5 but is it wise to take this to the limit?
In other pic of the schematic of that same fastAvrLink a pull up to Vcc can be seen in connected to the avr pins connected to the TI but again if Vp > Vcc current would flow to Vcc...
So i thought of something like
point A -> ?ohm resistor -> uC pin -> Zener4v7 -> ground
Again, any help here is much appreciated!