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Synchronous FIFO Flag Update Cycle

Last Updated: June 27, 2011

- What is the Flag Update Cycle?
- Must the FIFO be read to update the empty flag?
- Must the FIFO be written to for the full flag to be updated?


A flag update cycle (otherwise known as boundary latency cycle) refers to

the clock cycle which updates the synchronous flags at a boundary. The

empty flag gets updated by the read clock (RCLK) and the full flag gets

updated by the write clock (WCLK).


You don't have to read data from the FIFO to update /EF as long as you

keep the RCLK running at all times. The empty flag (/EF) will get updated

when the first piece of data is written into the FIFO regardless of whether

you're reading from the FIFO. When starting with an empty FIFO it essentially

takes two read clocks to read the first word from the device. The first read

clock rising edge updates the Empty Flag (assuming a write has been

performed). This update cycle occurs whether the read is enabled or not.

The second read clock rising edge (enabled) will read the first word from

the device. Without asserting the read enable (REN) a read will not be



The same applies to the full flag (/FF). Keeping the WCLK running at all

times will make sure the /FF is updated right away. When the device is

full, a write clock is needed to update the full flag (FF). In this case it takes

two write cycles to write into a device which just became not full.


This type of flag operation is necessary to ensure that the empty and full

flags will be valid and usable for a minimum of one clock cycle. This

architecture eliminates short flag pulses characteristic of an asynchronous

FIFO. This concept is the same for devices that have dual-function pins.

EF#/OR and AE# should be synchronized to CLKB, whereas FF#/IR and

AF# are synchronized to CLKA.

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