Interfacing with 2.5V logic?

milw

I've tried searching the forum for this, but only found how to interface with 5V logic. My question is, sine the Props logic high threshold is Vdd/2 (1.65V, from p16 of the Prop manual)- will it detect logic high from devices running at 2.5V? From what I've found so far, 2.5V devices have a high signal threshold between 1.7 and 2V, but I don't know how much of a standard that is?

cheers- Scott

Paul Baker

Worst case for the propeller Vih is 0.6Vdd which is 1.98V @ Vdd=3.3V, So there may be instances where the Propeller would not recognize the output of a 2.5V device.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

CJ

Paul, correct me if I'm wrong, but, wouldn't comparing the 2.5's worst case output voltage to the propeller's worst case input threshold make more sense than comparing the input thresholds of both units?

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milw

Or, would it be possible to run the Prop (protoboard) at 2.5V? otherwise, I'll have to rig some level shifter. The signals I'm after are about 2 MHz, and I'm not able to get full spec sheets on the microcontroller, its a proprietary camera DSP chip.

Paul Baker

No the Propeller will not run at 2.5V, CJ you're correct I missed that point. Scott, are you able to measure the Voh of the DSP chip or find it's value?

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

viskr

If the device you are interfacing to is 3.3 or 5V tolerant, most likely nothing is required.

As Prop Vih = 0.6*3.3 = 1.98, and typical 2.5V logic has a Voh of 2.3V for lightly loaded signal (from 74LCX data sheet)

To be sure you really would need the switching characteristics for the device.

If in doubt this same circuit (from SparkFun) would work (2.5V device on left, 3.3 on right)

http://www.sparkfun.com/tutorial/Sensor-Interfacing/ST-5.jpg