Usage of the Output Enable (OE) Signal in Synchronous FIFOs
Question: 1. If OE is high, what happens to the data bus? 2. If there is a valid read operation, but OE is high, will the next read operation be the same word or the following word in the FIFO? 3. If OE is high during reset, will the data bus remain in a high-Z state? 4. Is it okay to connect the OE pin to ground?
1. Some FIFOs have an Output Enable (OE#) signal associated with the reading port. This control signal is provided to three-state the data bus regardless of what else is occuring in the FIFO. More specifically, when OE# is enabled (low), data in the output register will be available on the data bus. When OE# is disabled (high), the FIFO's outputs are forced to a High-Z (high impedance) state.
2. OE# is an asynchronous signal which means it is not tied to a clock. The FIFO will react to the state of the OE# signal asynchronously. This means that even if there is a valid read operation on the reading port, if OE# is high, no data will come out. However, the read operation does occur. That means the next read operation will actually point to the next word in the FIFO.
3. When OE# is not asserted, all data outputs still remain in a high-impedence state, even when RS# is asserted. All data outputs go LOW after the falling edge of RS only if OE is asserted.
4.If the FIFO outputs are not connected to a bidirectional bus, it will be fine to have the OE# pin asserted at all times (tied to ground) since this pin only controls the tristate buffer at the outputs.