OPERATIONAL DESCRIPTION
Unlike earlier versions of FIFOs, the LH5481 and
LH5491 use dual-port Random-Access-Memory, write
and read pointers, and special control logic. The write
pointe r is incremented by the falling edge of the Shift In
(SI) signal, while the read pointer is incremented by the
falling edge of the Shift O ut ( SO) signal. Th e Input Ready
(IR) signal enables data writing to the FIFO. The Output
Ready (OR) signal indicates valid read information i s
available on the Data Output (DO) pins.
Resetting The FIFO
The FIFO must be rese t, upon power-up, using the
Master Reset (MR) s ig nal. This causes t he FI FO t o enter
an empty state, indicated by the Output Ready (OR) being
LOW and Input Ready (IR) being HIG H. All Data Outp ut
(DO) pins will be LOW in this stat e. The AFE flag will be
HIGH, and the HF flag will be LOW .
If Shi ft In (SI) is HIGH, when the Master Reset (MR)
signal is ended, then t he d ata on the Dat a Input (DI) pins
will be written into the FIFO , and Input Ready (IR ) will
return LOW until Shift In (SI) is brought LOW.
If Shift In (SI) is LOW when the Master Reset (MR) is
deasserte d, then Input Ready (IR) goes HIGH, but the
data on th e Data Input (DI ) pins does not enter the FIFO
until Shift In (SI) goes HIGH.
Shifting Data In
Data Input (DI) is shifted into the FIFO on the rising
edge of Shift In (SI). This loads input data into the FIFO,
and causes Input Ready (I R) to go LOW. When a fal ling
edge of Shift In (SI) occurs,the write pointer i ncrements
to the next word position, and Input Ready (IR) goes
HIGH, indicating that the FIFO is ready to accept new
data. When the FIFO is full, Input Ready (IR) remains
LOW after the negative edge of Shift In (SI) signal; Shift
Out (SO) action is requ ired to unload a word of data and
bring Input Ready (IR) HIGH. (See ‘Bubblethrough Co ndition’ des cript ion. )
Shifting Data Out
Data is shifted out of the FIFO on the falling edge of
Shift Out (SO). The read pointer increments to the next
word locat ion; FIFO dat a, if present, appear s on the Data
Output (DO) pins; and the Outpu t Ready (O R) signal goes
HIGH. If F IFO da ta is n ot present, Output Ready ( OR)
sta ys LOW , in dicating that the FIFO is empty; in this case ,
the last va lid dat a read from the FIFO remains on the Data
Output (DO) pins. When the FIFO is not empty, Output
Rea dy (OR) goes LOW after the rising edge of Shift Out
(SO). The previous dat a remains on the Data Output (DO)
pins until a falling edge of Shift Out (SO).
Fallthro ugh Condition
When the FI FO is empt y , a data word en tering through
the Shift In (SI) action follows one of two seq uence s.
If Shift Out (SO) is LOW, the data propagates to the
Data Ou tpu t (DO) pins; and Output Ready (O R) goes
HIGH and stays HIGH until the next rising edge of Shift
Out (SO).
If Shift Out (SO) is held HIGH while data is shifted into
an emp ty FIFO as occurs i n depth cascading of FIFOs,
dat a propagat es to the Data Outpu t (DO) pins, and Outpu t
Read y ( OR) pulses HIGH for a minimum time duration
specified by t
POR
and then goes back LOW again. The
stor ed word remains on the Data Output (DO) pins. If
mor e words are writt en int o the FIFO, the y line up behind
the fir st word, and do not app ear on the Dat a Outp ut (DO)
pins until Shift Out (SO) has return ed LO W .
Bubblethrough Condition
When the FIF O is full, Shift O ut (SO) action initiates
one of the following two sequences:
If Shif t In (SI ) is LOW , Input Ready (IR) goes HIGH and
sta ys HIG H unt il the next rising edge of Shift In (SI) .
If Shift In (SI) is held HIGH while data is shifted out of
a full FIFO , as occur s in dept h cascading of FIF Os , Input
Ready (IR) pulses HIGH for a minimum time duration
specif ied by t
PIR
, and then goes back LOW again. Special
Data Input (DI) setup and hold times (t
SIR
and t
HIR
,
respectively) are defined for this condition.
LH5481/91 64 × 8 / 64 × 9 F IFO
6