A FIFO queue is a first-in, first-out digital device that allows two parties to communicate with a channel of limited size by following specific rules: one party writes, the other reads. The first thing written will be the first thing read. Reading an empty queue is disallowed and writing to a full queue is disallowed. Empty and full status can be checked via the proper status pin before use. In psuedo-code, two parties can communicate with this FIFO as follows:
Party A
while !full:
write_entry(item)
Party B
while !empty:
item = read_entry()
The queue works in First-Word Fall-Through mode meaning that the top item is always available on the read bus even
if you haven’t set read_request high. If you want to see the next item in the queue on your next read, be sure
to set read_request high.
almost_full and almost_empty signals exist so you can batch reads and writes. Instead of checking for
full or empty on each read or write you can instead check almost_full or almost_empty and batch read or writes
based on how many slots are available. For this design taped out in TinyTapeout 4, almost_full means 12 of 16 slots
have been used and almost_empty means that 12 of 16 slots are free.
Write an item, read an item. Check the status bits. See the unit tests for more ideas!
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | item[0] | item[0] | empty (read-only) |
| 1 | item[1] | item[1] | full (read-only) |
| 2 | item[2] | item[2] | underflow (read-only) |
| 3 | item[3] | item[3] | overflow (read-only) |
| 4 | item[4] | item[4] | almost_empty (read-only) |
| 5 | item[5] | item[5] | almost_full (read-only) |
| 6 | item[6] | item[6] | write_enable (set this high to write a value) |
| 7 | item[7] | item[7] | read_enable (set this high to read the latest entry from the FIFO) |