Level shifting with a reverse biased small signal diode

[hackits]

I have found a circuit that uses a BAS16 small signal diode to level shift a 5V (or 3.3V) TTL signal down to a 1.8V TTL signal. The cathode of the diode is on the high voltage side and the anode is on the low voltage side, so it looks like it is backwards compared to the direction of current flow. http://i.imgur.com/Ke5Cv.jpg".

I imagine the 1.8V input pin is high-z so very little current will sink into the pin already, much less anymore with the small current going through the reversed diode.

Has anyone seen this done before or knows how this works?

Thanks

 

[Fish4Fun]

If the 1.8V side is an input, Hi-Z w/Pull-Up, then the diode only allows the 3.3V output to pull the 1.8V input low.

If the 1.8V side is an input, Hi-Z with NO Pull-Up then you would need to add a Pull-Up to the 1.8V Vcc.

If the 1.8V side is an Output, it will have no affect on the 3.3V line.

Fish

 

[Bob S]

TTL logic is a current sinking logic, not current sourcing. A TTL input requires a -1.6 mA current sink for a logic low (to pull down the input NPN emitter), and a few microamps for a logic high. Datasheet specs state that a logic low is less than 0.8 volts, and a logic high more than 2 volts. Normal operation is 0.4 volts (low) and 3 volts (high).

See thumbnail schematic for TTL totem pole and open collector outputs. Note that the input is an NPN emitter with the base pulled up to 5 volts through a 4k resistor.

Bob S