184.108.40.206. Bidirectional Configuration
The bidirectional path consists of two data flow paths:
- Input path active
- Output path active
When the input path is active, the output enable disables the tri-state buffer, which prevents data from being sent out on the output path. Disabling the tri-state buffer prevents contention at the I/O pin. The input path behaves like the input configuration as shown in Figure 3–1 on page 3–1. When the output path is active, the output enable register AOE controls the flow of data from the output registers. During outgoing transactions, the bidirectional configuration behaves like the output configuration as shown in Figure 3–3 on page 3–3. The second output enable register (BOE) is used for DDR SDRAM interfaces. This negative-edge register extends the high-impedance state of the pin by a half clock cycle. This option is useful to provide the write preamble for the DQS strobe in the DDR SDRAM interfaces. This feature is enabled by using the Delay switch-on by a half clock cycle option in the ALTDDIO_BIDIR IP core in the Quartus II software. You can bypass the input registers and latch to get a combinational output (combout) from the pin going into the APEX™ II or Stratix® series device. Furthermore, the input data ports (dataout_h and dataout_l) can be disabled. These features are especially useful for generating data strobes like DQS.
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