Hi,

I have this circuit and I wanted to see if this can drive a logic low here. The output is 2.5V LVCMOS and the input is 1.8V LVCMOS.

R1 is a strapping resistor used on power up to detemine the state of the interface (needed). R3 is an internal pulldown.

I added R2 and R4 to bring the level down from 2.5 to 1.8V.

Thanks!

 

Taking R1 out of the circuit, that leaves a 2.5 V LVCMOS output trying to drive a 95 ohm load, for an output current of over 26 mA. Without knowing more about the two devices, that seems unnecessarily high. Also, many LVCMOS devices have inputs that can tolerate higher voltages. Does yours?

ak

 

I agree that the extremely low values for R2 and R4 are a dead short to a low voltage Cmos output. Instead, use 27k and 68k or 270k and 680k.

 

I wanted to see if this can drive a logic low here. The output is 2.5V LVCMOS and the input is 1.8V LVCMOS.

Part numbers?

 

Taking R1 out of the circuit, that leaves a 2.5 V LVCMOS output trying to drive a 95 ohm load, for an output current of over 26 mA. Without knowing more about the two devices, that seems unnecessarily high. Also, many LVCMOS devices have inputs that can tolerate higher voltages. Does yours?

ak

The max input voltage is only +/- 125mV above the rail which is 1.8 so it looks like we cannot drive this input up to 2.5V

 

I agree that the extremely low values for R2 and R4 are a dead short to a low voltage Cmos output. Instead, use 27k and 68k or 270k and 680k.

So even with those values as suggest, wouldn't the voltage divider be such that it is 10K and 98K which would leave the input biased at 2.2V? Would the gate be able to drive it low? And the 2.2V would be too high for the input to handle with the 10K strapping resistor.

 

A Cmos input draws no current. Then the almost 2.5V high into the 95k voltage divider produces 1.78V at the input of the 1.8V powered device as a high and produces 0V as a low. The 10k pullup is not needed.

 

Is the 2.5 V output a totem pole? That is, can it both source (high state) and sink (low state) current?

ak

 

A Cmos input draws no current. Then the almost 2.5V high into the 95k voltage divider produces 1.78V at the input of the 1.8V powered device as a high and produces 0V as a low. The 10k pullup is not needed.

The 10K pullup is needed as a strapping resistor to tell the device which mode to operate in

 

Is the 2.5 V output a totem pole? That is, can it both source (high state) and sink (low state) current?

ak

Yes it can sink and source current