Should I use pull up resistors in this configuration?

Question

I am interfacing a 3.3V FPGA to a 5V DSP. I am using this bidirectional voltage transceiver: http://www.nxp.com/documents/data_sheet/GTL2000.pdf

In the diagram in page 4 they're using pull up resistors at the high voltage side and they state:

The processor output can be totem pole or open drain (pull up resistors may be required) and the chipset output can be totem pole or open drain (pull up resistors are required to pull the Dn outputs to VCC).

The DSP I'm using is TI TMS320C31 and will be interfaced to the FPGA Cyclone EP1C6Q240C8N.

Should I use pull up resistors on one of the sides or both or I don't even need them?

Answer 1

For the outputs you configure as open-drain you'd need the pull-up. Not if you have push-pull outputs. Open-drain is typically used for wired-ORing outputs, you usually don't use them for a direct output-to-input connection. Push-pull outputs are faster because the low impedance allows for fast edges, while the asymmetrical output of an open-drain will give you slower positive edges.

In the schematic of page 4 open-drains are used to allow different output voltages, depending on the voltage level the pull-up is connected to.