7

COMMERCIALANDINDUSTRIAL

TEMPERATURERANGES

IDT72281/72291

CMOS SuperSync FIFO™ 65,536 x 9 and 131,072 x 9

When configured in IDT Standard mode, the

EF and FF outputs are

double register-buffered outputs.

Relevant timing diagrams for IDT Standard mode can be found in Figure

7, 8 and 11.

FIRST WORD FALL THROUGH MODE (FWFT)

In this mode, the status flags,

IR, PAF, HF, PAE, and OR operate in the

manner outlined in Table 2. To write data into to the FIFO,

WEN must be

LOW. Data presented to the DATA IN lines will be clocked into the FIFO on

subsequent transitions of WCLK. After the first write is performed, the Output

Ready (

OR) flag will go LOW. Subsequent writes will continue to fill up the

FIFO.

PAE will go HIGH after n + 2 words have been loaded into the FIFO,

where n is the empty offset value. The default setting for this value is stated

in the footnote of Table 2. This parameter is also user programmable. See

section on Programmable Flag Offset Loading.

If one continued to write data into the FIFO, and we assumed no read

operations were taking place, the

HF would toggle to LOW once the 32,770th

word for the IDT72281 and 65,538th word for the IDT72291, respectively

was written into the FIFO. Continuing to write data into the FIFO will cause

the

PAF to go LOW. Again, if no reads are performed, the PAF will go LOW

after (65,537-m) writes for the IDT72281 and (131,073-m) writes for the

IDT72291, where m is the full offset value. The default setting for this value

is stated in the footnote of Table 2.

When the FIFO is full, the Input Ready (

IR) flag will go HIGH, inhibiting

further write operations. If no reads are performed after a reset,

IR will go

HIGH after D writes to the FIFO. D = 65,537 writes for the IDT72281 and

131,073 writes for the IDT72291, respectively. Note that the additional word

in FWFT mode is due to the capacity of the memory plus output register.

If the FIFO is full, the first read operation will cause the

IR flag to go LOW.

Subsequent read operations will cause the

PAF and HF to go HIGH at the

conditions described in Table 2. If further read operations occur, without

write operations, the

PAE will go LOW when there are n + 1 words in the

FIFO, where n is the empty offset value. Continuing read operations will

cause the FIFO to become empty. When the last word has been read from

the FIFO,

OR will go HIGH inhibiting further read operations. REN is ig-

nored when the FIFO is empty.

When configured in FWFT mode, the

OR flag output is triple register-

buffered, and the

IR flag output is double register-buffered.

Relevant timing diagrams for FWFT mode can be found in Figure 9, 10

and 12.

PROGRAMMING FLAG OFFSETS

Full and Empty Flag offset values are user programmable. The IDT72281/

72291 has internal registers for these offsets. Default settings are stated in

the footnotes of Table 1 and Table 2. Offset values can be programmed into

the FIFO in one of two ways; serial or parallel loading method. The selec-

tion of the loading method is done using the

LD (Load) pin. During Master

Reset, the state of the

LD input determines whether serial or parallel flag

offset programming is enabled. A HIGH on

LD during Master Reset selects

serial loading of offset values and in addition, sets a default

PAE offset value

of 3FFH (a threshold 1,023 words from the empty boundary), and a default

PAF offset value of 3FFH (a threshold 1,023 words from the full boundary).

A LOW on

LD during Master Reset selects parallel loading of offset values,

and in addition, sets a default

PAE offset value of 07FH (a threshold 127

words from the empty boundary), and a default

PAF offset value of 07FH (a

threshold 127 words from the full boundary). See Figure 3, Offset Register

Location and Default Values.

FUNCTIONAL DESCRIPTION

TIMING MODES: IDT STANDARD VS FIRST WORD FALL THROUGH

(FWFT) MODE

The IDT72281/72291 support two different timing modes of operation:

IDT Standard mode or First Word Fall Through (FWFT) mode. The selec-

tion of which mode will operate is determined during Master Reset, by the

state of the FWFT/SI input.

If, at the time of Master Reset, FWFT/SI is LOW, then IDT Standard mode

will be selected. This mode uses the Empty Flag (

EF) to indicate whether or

not there are any words present in the FIFO. It also uses the Full Flag

function (

FF) to indicate whether or not the FIFO has any free space for

writing. In IDT Standard mode, every word read from the FIFO, including

the first, must be requested using the Read Enable (

REN) and RCLK.

If, at the time of Master Reset, FWFT/SI is HIGH, then FWFT mode will

be selected. This mode uses Output Ready (

OR) to indicate whether or not

IR) to

indicate whether or not the FIFO has any free space for writing. In the

FWFT mode, the first word written to an empty FIFO goes directly to Qn

after three RCLK rising edges,

REN = LOW is not necessary. Subsequent

words must be accessed using the Read Enable (

REN) and RCLK.

Various signals, both input and output signals operate differently depend-

ing on which timing mode is in effect.

IDT STANDARD MODE

In this mode, the status flags,

FF, PAF, HF, PAE, and EF operate in the

manner outlined in Table 1. To write data into to the FIFO, Write Enable

(

WEN) must be LOW. Data presented to the DATA IN lines will be clocked

into the FIFO on subsequent transitions of the Write Clock (WCLK). After the

first write is performed, the Empty Flag (

EF) will go HIGH. Subsequent

writes will continue to fill up the FIFO. The Programmable Almost-Empty flag

(

PAE) will go HIGH after n + 1 words have been loaded into the FIFO,

where n is the empty offset value. The default setting for this value is stated

in the footnote of Table 1. This parameter is also user programmable. See

section on Programmable Flag Offset Loading.

If one continued to write data into the FIFO, and we assumed no read

operations were taking place, the Half-Full flag (

HF) would toggle to LOW

once the 32,769th word for IDT72281 and 65,537th word for IDT72291

respectively was written into the FIFO. Continuing to write data into the

FIFO will cause the Programmable Almost-Full flag (

PAF) to go LOW.

Again, if no reads are performed, the

PAF will go LOW after (65,536-m)

writes for the IDT72281 and (131,072-m) writes for the IDT72291. The

offset “m” is the full offset value. The default setting for this value is stated in

the footnote of Table 1. This parameter is also user programmable. See

section on Programmable Flag Offset Loading.

When the FIFO is full, the Full Flag (

FF) will go LOW, inhibiting further write

operations. If no reads are performed after a reset,

FF will go LOW after D

writes to the FIFO. D = 65,536 writes for the IDT72281 and 131,072 for the

IDT72291, respectively.

If the FIFO is full, the first read operation will cause

FF to go HIGH.

Subsequent read operations will cause

PAF and HF to go HIGH at the

conditions described in Table 1. If further read operations occur, without

write operations,

PAE will go LOW when there are n words in the FIFO,

where n is the empty offset value. Continuing read operations will cause the

FIFO to become empty. When the last word has been read from the FIFO,

the

EF will go LOW inhibiting further read operations. REN is ignored when

the FIFO is empty.