Why does the Quartus II software convert my bidirectional pin to an output-only pin? - Why does the Quartus II software convert my bidirectional pin to an output-only pin? Description The Quartus® II software version 6.0 and later synthesizes VHDL directional signals more strictly. Because it enforces the VHDL rules, the Quartus II software converts incorrectly-coded bidirectional buffers to output pins. For example, if you implement a bidirectional buffer as shown in the following code, the Quartus II software synthesizes an output pin (beginning with version 6.0). process(mdio_out,mdio_oe) begin if (mdio_oe = '1') then mac_mdio <= mdio_out; else mac_mdio <= 'Z'; end if end process; mdio_oe <= NOT(mdio_oe_n); test_out <= mac_mdio and test; mii_mdio <= mac_mdio; If mac_mdio is a signal and mii_mdio is the bidirectional port, the last assignment above is directional. As written, it is impossible for data to flow from mii_mdio to mac_mdio. Beginning with version 6.0, the Quartus II software enforces the directional nature of this VHDL assignment by inserting a buffer between mac_mdio and mii_mdio. This buffer appears in the RTL Viewer as a directional buffer in series with the tri-state buffer, and the intended bidirectional pin (mii_mdio) is synthesized as an output only. Earlier versions of the Quartus II software do not enforce the directional nature of these types of assignments, synthesizing the code above as a bidirectional pin. To conform to the VHDL standard, remove the last signal assignment statement in the example above and write the code as follows: process(mdio_out,mdio_oe) begin if (mdio_oe = '1') then mii_mdio <= mdio_out; else mii_mdio <= 'Z'; end if; end process; mdio_oe <= NOT(mdio_oe_n); test_out <= mii_mdio and test; Custom Fields values: ['novalue'] Troubleshooting novalue False ['novalue'] ['novalue'] novalue novalue ['Programmable Logic Devices'] ['novalue'] ['novalue'] ['novalue']