OE Pin Technique
Basic Blocks and Functions
This technique can be used within any GreenPAK with OE pins.
Typically GreenPAK I/O can be configured as an input or output. Output enable (OE) pins are select pins within most GreenPAK that allow the pin to dynamically change between a Digital input and Digital output (Figure A). When the OE GPIO is set as a permanent input the OE pin is set to ground and if the GPIO is set as a permanent output the OE pin is set to VDD. Setting the GPIO as a digital input/output allows for this selection to be made in the matrix.
Setting a GPIO as a digital input/output allows for two-way communication. It also allows for the GPIO to be set to Hi-Z in addition to a logical high and low.
If the GPIO is used for two-way communication, it’s important to implement a timing circuit for OE selection. In the example circuit below the OE pins of CLK_IO and Data_IO are switched from low to high after CNT2 sees 8 clocks, consequently setting the OE pins as outputs to transmit the internal signals. After another 8 clocks the OE pins are reset low, setting them as inputs again to receive an external signal.