OPERATIONAL DESCRIPTION

Reset

The LH540203 is reset whenever the Reset input (RS)

is taken LOW. A reset operation initializes both the read-

address pointer and the write-address pointer to point to

location zero, the first physical memory location. During

a reset operation, the state of the XI and FL/RT inputs

determines whether the device is in standalone mode or

in depth-cascaded mode. (See Tables 1 and 2.) The reset

operation forces the Empty Flag EF to be asserted

(EF = LOW), and the Half-Full Flag HF and the Full Flag

FF to be deasserted (HF = FF = HIGH); the Data Out pins

A reset operation is required whenever the LH540203

first is powered up. The Read (R) and Write (W) inputs

may be in any state when the reset operation is initiated;

but they must be HIGH, before the reset operation is

Write

A write cycle is initiated by a falling edge of the Write

(W) control input. Data setup times and hold times must

tions may occur independently of any ongoing read op-

erations. However, a write operation is possible only if the

FIFO is not full, (i.e., if the Full Flag FF is HIGH).

At the falling edge of W for the first write operation after

the memory is half filled, the Half-Full Flag is asserted

(HF = LOW). It remains asserted until the difference

between the write pointer and the read pointer indicates

that the data words remaining in the LH540203 are filling

the FIFO memory to less than or equal to one-half of its

total capacity. The Half-Full Flag is deasserted

(HF = HIGH) by the appropriate rising edge of R. (See

Table 3.)

The Full Flag is asserted (FF = LOW) at the falling edge

of W for the write operation which fills the last available

location in the FIFO memory array. FF = LOW inhibits

further write operations until FF is cleared by a valid read

operation. The Full Flag is deasserted (FF = HIGH) after

the next rising edge of R releases another memory loca-

tion. (See Table 3.)

Read

A read cycle is initiated by a falling edge of the Read

(R) control input. Read data becomes valid at the data

R. After R goes HIGH, the data outputs return to a

high-impedance state. Read operations may occur inde-

pendently of any ongoing write operations. However, a

read operation is possible only if the FIFO is not empty

(i.e., if the Empty Flag EF is HIGH).

The LH540203’s internal read-address and write-

address pointers operate in such a way that consecutive

read operations always access data words in the same

order that they were written. The Empty Flag is asserted

(EF = LOW) after that falling edge of R which accesses

the last available data word in the FIFO memory. EF is

deasserted (EF = HIGH) after the next rising edge of W

loads another valid data word. (See Table 3.)

Data Flow-Through

Read-data flow-through mode occurs when the Read

(R) control input is brought LOW while the FIFO is empty,

and is held LOW in anticipation of a write cycle. At the end

of the next write cycle, the Empty Flag EF momentarily is

deasserted, and the data word just written becomes

while the R input remains LOW; but only data from the

first write operation flows through to the data outputs.

Additional data words, if any, may be accessed only by

toggling R.

Write-data flow-through mode occurs when the Write

(W) input is brought LOW while the FIFO is full, and is

held LOW in anticipation of a read cycle. At the end of the

read cycle, the Full Flag momentarily is deasserted, but

then immediately is reasserted in response to W being

held LOW. A data word is written into the FIFO on the

rising edge of W, which may occur no sooner than

PIN DESCRIPTIONS

PIN

PIN TYPE

1

DESCRIPTION

I

Input Data Bus

O/Z

Output Data Bus

W

I

Write Request

R

I

Read Request

EF

O

Empty Flag

FF

O

Full Flag

PIN

PIN TYPE

1

DESCRIPTION

XO/HF

O

Expansion Out/Half-Full Flag

XI

I

Expansion In

FL/RT

I

First Load/Retransmit

RS

I

Reset

V

Positive Power Supply

V

Ground

NOTE:

1.

I = Input, O = Output, Z = High-Impedance, V = Power Voltage Level

CMOS 2048

× 9 Asynchronous FIFO

LH540203

3