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LH540215 Datasheet(PDF) 20 Page - Sharp Corporation

Part # LH540215
Description  512 x 18 / 1024 x 18 Synchronous FIFO
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Manufacturer  SHARP [Sharp Corporation]
Direct Link  http://sharp-world.com/
Logo SHARP - Sharp Corporation

LH540215 Datasheet(HTML) 20 Page - Sharp Corporation

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If the FIFO has been reset by asserting RS (LOW), and
no read operations have been performed since the
completion of the reset operation, PAF goes LOW after
(512-p) write operations for the LH540215, or after
(1024-p) write operations for the LH540225. (See
Table 4.)
If p is still at its default value, PAF is LOW whenever
the FIFO is from seven-eighths full to completely full.
In the IDT-Compatible Operating Mode, PAF changes
from HIGH to LOW only after a LOW-to-HIGH transition
of the Write Clock WCLK, and from LOW to HIGH only
after a LOW-to-HIGH transition of the Read Clock RCLK.
Thus, in this operating mode, PAF behaves as an ‘asyn-
chronous flag.’
In the Enhanced Operating Mode, on the other
hand, PAF gets updated only after a LOW-to-HIGH
transition of the Write Clock WCLK, and thus behaves
as a ‘synchronous flag,’ whenever Control Register
bit 04 is HIGH. (See Table 5.)
WRITE EXPANSION OUT/HALF-FULL FLAG
(WXO/HF)
WXO/HF is a dual-purpose signal. In ‘standalone’ op-
eration, it behaves as a Half-Full Flag (HF), in accordance
with Table 4. In IDT-compatible ‘cascaded’ operation, it
behaves as a Write Expansion Output (WXO) signal to
coordinate writing operations with the next FIFO in the
cascade. Under these same conditions, also, the dual-
purpose WXI/
WEN2 and RXI/REN2 inputs behave as
Write Expansion Input (WXI) and Read Expansion Input
(RXI) signals respectively.
When two or more LH540215 or LH540225 FIFOs are
‘cascaded’ to operate as a deeper ‘effective FIFO,’ in an
IDT-style ‘daisy-chain’ ring configuration, the Write Ex-
pansion Input (WXI) of each FIFO is connected to WXO
of the previous FIFO in the ring, with WXI of the ‘first-load’
or ‘master’ FIFO being connected to WXO of the last FIFO
so as to complete the ring. Similar connections are made
for each FIFO in the ring, parallel to these WXO-to-WXI
connections, for Read Expansion Input (RXI) and Read
Expansion Output (RXO/
EF2, when it is behaving as
RXO).
When the last physical location has been written in a
FIFO operating in cascaded mode, a LOW-going pulse is
emitted by that FIFO on its WXO output, and the FIFO is
deactivated for writing at the next valid WCLK; and the
next FIFO in the ring is simultaneously activated for
writing. Otherwise, WXO remains constantly HIGH when-
ever the FIFO is operating in cascaded mode. This LOW-
going WXO pulse serves as a ‘write token’ in the
‘token-passing’ FIFO-cascading scheme; it is passed on
to the next FIFO in the ring via its WXI input. When this
next FIFO receives the write token, it is activated for
writing at the next valid WCLK.
The foregoing description applies both to the ‘first-load’
or ‘master’ FIFO in the ring, and to any and all ‘slave’
FIFOs in the ring. However, WXO has no necessary
function for FIFOs operating in the ‘standalone’ mode.
Consequently, in that mode, the same output pin is used
for HF; it follows that HF is not available as an output from
any FIFO which is operating in the IDT-compatible cas-
caded mode. A FIFO is initialized into ‘cascaded master’
mode, into ‘cascaded slave’ mode,
into interlocked-par-
alleled mode, or into standalone mode according to the
state of its WXI/
WEN2, RXI/REN2, and FL/RT control
inputs during a reset operation,
and of EMODE. (See
Table 1, Table, 2, and Table 5.)
In standalone
or interlocked-paralleled operation,
HF goes LOW whenever the FIFO is more than half full;
that is, whenever subtracting the value of the FIFO’s
internal read pointer from the value of its internal write
pointer yields a difference which is less than half of the
total nominal number of 18-bit words in the FIFO’s physi-
cal memory, which is 256 for the LH540215 or 512 for the
LH540225 respectively. (See Table 4.) The subtraction is
performed using modular arithmetic, modulo this total
nominal number of words, which is 512 for the LH540215
or 1024 for the LH540225 respectively.
If the FIFO has been reset by asserting RS (LOW), and
it is operating in standalone mode
or in interlocked-par-
alleled mode, and no read operations have been per-
formed since the completion of the reset operation, HF
goes LOW after 257 write operations for the LH540215,
or after 513 write operations for the LH540225. (See
again Table 4.)
In the IDT-Compatible Operating Mode, HF changes
from HIGH to LOW only after a LOW-to-HIGH transition
of the Write Clock WCLK, and from LOW to HIGH only
after a LOW-to-HIGH transition of the Read Clock RCLK.
Thus, in this operating mode, HF behaves as an ‘asyn-
chronous flag.’
In the Enhanced Operating Mode, on the other
hand, HF gets updated only after a LOW-to-HIGH
transition of the Read Clock RCLK, or else after a
LOW-to-HIGH transition of the Write Clock WCLK,
according to the setting of bits 03 and 02 of the
Control Register (see Table 5). Thus, in this mode HF
behaves as a ‘synchronous flag,’ and may be syn-
chronized either to the input side of the FIFO (i.e., to
WCLK), or to the output side of the FIFO (i.e., to
RCLK).
PROGRAMMABLE ALMOST-EMPTY FLAG (PAE)
PAE goes LOW whenever the FIFO is ‘almost empty’;
that is, whenever subtracting the value of the FIFO’s
internal write pointer from the value of its internal read
pointer yields a difference which is less than q + 1, where
‘q’ is the value of the Programmable-Almost-Empty-Flag
Offset. The subtraction is performed using modular arith-
metic, modulo the total nominal number of 18-bit words
BOLD ITALIC = Enhanced Operating Mode
LH540215/25
512 x 18/1024 x 18 Synchronous FIFO
20


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