Answers Database

How can the 4000XL/XV device talk to a 5V CMOS level device?


Record #2784

Product Family:  Hardware

Product Line:  4000XL

Problem Title:
How can the 4000XL/XV device talk to a 5V CMOS level device?


Problem Description:
Keywords: 3.3V, CMOS, 4000XL, XC4000XV, 5V


Urgency: Standard

Description

The 4000XL/XV devices are unable to talk to 5 CMOS devices
simply because there is no way an output signal may reach a
legal CMOS high (70% of VCC which means 3.5V).

However there is a simple workaround that will allow these
devices to communicate with 5V CMOS level devices.


Solution 1:

instead driving the outputs to an OBUF,  use an OBUFT.
Tie the signal to the T input of the OBUFT and the I input to GND,  and then
add an external 1K resistor to the pins.
The speed of the swing rate of the signal increases as the
external resistor decreases. However, users should be careful Ioh does
not go higher in magnitude than 4ma. That is, the external
resistor cannot be smaller than 800ohms.



End of Record #2784