AR# 2784: How can the 4000XL/XV device talk to a 5V CMOS level device?
How can the 4000XL/XV device talk to a 5V CMOS level device?
Keywords: 3.3V, CMOS, 4000XL, XC4000XV, 5V
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.
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.