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LH540215

512 × 18 / 1024 × 18 Synchronous FIFO

Sharp Electronics 

LH540215/25

512 x 18/1024 x 18 Synchronous FIFO

READ CLOCK (RCLK)

A rising edge (LOW-to-HIGH transition) of RCLK initi-

ates a FIFO read cycle if LD is HIGH, or a programma-

ble-register read cycle if LD is LOW. All output-side

synchronous control inputs must meet setup and hold

times with respectto the rising edge of RCLK. The 18 data

outputs, and the output-side status flags, are meaningful

after specified time intervals, following a rising edge of

RCLK.

Conceptually, the RCLK input receives a free-running,

periodic ‘clock’ waveform, which is used to control other

signals which are edge-sensitive. However, there actually

is not any absolute requirement that the RCLK waveform

must be periodic. An ‘asynchronous’ mode of operation is

in fact possible, if REN is continuously asserted (that is,

is continuously held LOW), and RCLK receives aperiodic

‘clock’ pulses of suitable duration. There likewise is no

requirement that RCLK must have any particular synchro-

nization relation to the write clock WCLK. These two clock

inputs may in fact receive the same ‘clock’ signal; or they

may receive totally-different signals, which are not syn-

chronized to each other in any way.

READ ENABLE (REN)

Whenever REN is being asserted (is LOW), and the

FIFO is not empty, an 18-bit data word is loaded into the

output register from the memory array at every RCLK

rising edge (LOW-to-HIGH transition). Data words are

read from the two-portmemory array sequentially, regard-

less of any ongoing write operation. Whenever REN is not

being asserted (is HIGH), the output register retains

whatever data word it contained previously, and no new

data word gets loaded into it from the memory array.

To prevent underrunning the internal FIFO boundaries,

further read operations are inhibited whenever the Empty

Flag (EF) is being asserted (is LOW). If a valid write

operation then occurs, upon the completion of that write

cycle EF again goes HIGH after a time tREF, and another

read operation is allowed to begin whenever RCLK

makes another LOW-to-HIGH transition. Effectively, REN

is overridden by EF; thus, during normal FIFO operation,

REN has no effect when the FIFO is empty.

In the Enhanced Operating Mode, one (or, some-

times two) additional read-enable inputs may be

ANDed with REN to control reading, depending on

the state of Control-Register Bit 05. The additional

read-enable input(s) are REN2 (and OE).

Whenever EMODE is being asserted (is LOW),

RXI/REN2 functions as REN2, an additional duplicate

(albeit assertive-HIGH) Read-Enable input, in order to

provide an ‘interlocking’ mechanism for reliable

synchronization of two paralleled FIFOs.

Also, if Control Register bit 05 has been set, OE

takes on the extra role of serving as yet another

duplicate read-enable input, in addition to its usual

function of controlling the FIFO’s data outputs, in

order to inhibit further read operations whenever the

FIFO’s data outputs are disabled, and thereby to

prevent data loss under some circumstances.

OUTPUT ENABLE (OE)

OE is an assertive-LOW, asynchronous, output

enable. In the IDT-Compatible Operating Mode, OE has

only the effect of enabling or disabling the data outputs

Q0 – Q17. That is, disabling Q0 – Q17 does not inhibit a

read operation, for data being transmitted to the output

register; the same data will remain available later, when

the outputs are again enabled, unless subsequently over-

written. When Q0 – Q17 are enabled, each of these 18

data outputs is in a normal HIGH or LOW state, according

to the bit pattern of the data word in the output register.

When Q0 – Q17 are disabled, each of these outputs is in

the high-Z (high-impedance) state.

In the Enhanced Operating Mode, if Control Regis-

ter bit 05 has been set, OE behaves as an additional

read-enable control input, as well as enabling and

disabling the data outputs Q0 – Q17. Under these

circumstances, incrementing the read-address

pointer is inhibited whenever Q0 – Q17 are in the

high-Z state. Thus, ‘reading’ successive words which

fail ever to reach the outputs is prevented, as a

safeguard against data loss.

LOAD (LD)

The Sharp LH540215/25 FIFOs contain three 18-bit

programmable registers. The contents of these three

registers may be loaded with data from the data inputs

D0 – D17, or read out onto the data outputs Q0 – Q17. The

first two registers are the Programmable-Flag-Offset-

Value Registers, for the Programmable Almost-Empty

Flag (PAE) and the Programmable Almost-Full Flag(PAF)

respectively. The third register is the Control Register,

which includes several configuration-control bits

for selectively enabling and disabling Sharp’s

Enhanced-Operating-Mode features.

None of these three registers makes use of all of its

available 18 bits. Figure 5 shows which bit positions of

each register are operational. The two Programmable-

Flag-Offset-Value Registers each contain an offset value

in bits 0-8 (LH540215) or bits 0 – 9 (LH540225); bits 9 –

17 (LH540215) or bits 10 – 17 (LH540225) are unused.

The default values for both offsets are one-eighth of the

total number of words in the FIFO memory array, minus

one: 63 for a 512 × 18 FIFO, and 127 for a 1024 × 18

FIFO.

BOLD ITALIC = Enhanced Operating Mode

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