15
COMMERCIALTEMPERATURERANGE
IDT723624/723634/723644 CMOS SyncBiFIFO WITH BUS-MATCHING
256 x 36 x 2, 512 x 36 x 2, 1,024 x 36 x 2
The Mail2 Register Flag (
MBF2)issetHIGHbyaLOW-to-HIGHtransition
on CLKA when a Port A read is selected by
CSA, W/RA, and ENA with MBA
HIGH.
For a 36-bit bus size, 36 bits of mailbox data are placed on A0-A35. For
an 18-bit bus size, 18 bits of mailbox data are placed on A0-A17. (In this case,
A18-A35 are indeterminate.) For a 9-bit bus size, 9 bits of mailbox data are
placed on A0-A8. (In this case, A9-A35 are indeterminate.)
The data in a mail register remains intact after it is read and changes only
whennewdataiswrittentotheregister.TheEndianselectfeaturehasnoeffect
on mailbox data. For mail register and Mail Register flag timing diagrams, see
Figure 27 and 28.
BUS SIZING
The Port B bus can be configured in a 36-bit long word, 18-bit word, or
9-bit byte format for data read from FIFO1 or written to FIFO2. The levels
applied to the Port B Bus Size select (SIZE) and the Bus-Match select (BM)
determine the Port B bus size. These levels should be static throughout FIFO
operation.BothbussizeselectionsareimplementedatthecompletionofMaster
Reset, by the time the Full/Input Ready flag is set HIGH, as shown in Figure
2.
Two different methods for sequencing data transfer are available for Port
B when the bus size selection is either byte- or word-size. They are referred
to as Big-Endian (most significant byte first) and Little-Endian (least significant
bytefirst).ThelevelappliedtotheBig-Endianselect(BE)inputduringtheLOW-
to-HIGH transition of
MRS1andMRS2selectstheendianmethodthatwillbe
active during FIFO operation. BE is a don’t care input when the bus size
selected for Port B is long word. The endian method is implemented at the
completion of Master Reset, by the time the Full/Input Ready flag is set HIGH,
as shown in Figure 2.
Only 36-bit long word data is written to or read from the two FIFO memories
on the IDT723624/723634/723644. Bus-matching operations are done after
data is read from the FIFO1 RAM and before data is written to the FIFO2 RAM.
These bus-matching operations are not available when transferring data via
mailbox registers. Furthermore, both the word- and byte-size bus selections
limit the width of the data bus that can be used for mail register operations. In
this case, only those byte lanes belonging to the selected word- or byte-size
bus can carry mailbox data. The remaining data outputs will be indeterminate.
The remaining data inputs will be don’t care inputs. For example, when a word-
size bus is selected, then mailbox data can be transmitted only between A0-
A17 and B0-B17. When a byte-size bus is selected, then mailbox data can be
transmitted only between A0-A8 and B0-B8. (See Figures 27 and 28).
BUS-MATCHING FIFO1 READS
Data is read from the FIFO1 RAM in 36-bit long word increments. If a long
word bus size is implemented, the entire long word immediately shifts to the
FIFO1 output register. If byte or word size is implemented on Port B, only the
firstoneortwobytesappearontheselectedportionoftheFIFO1outputregister,
withtherestofthelongwordstoredinauxiliaryregisters.Inthiscase,subsequent
FIFO1 reads output the rest of the long word to the FIFO1 output register in the
order shown by Figure 2.
When reading data from FIFO1 in byte or word format, the unused B0-B35
outputsareindeterminate.
BUS-MATCHING FIFO2 WRITES
DataiswrittentotheFIFO2RAMin36-bitlongwordincrements.Datawritten
toFIFO2withabyteorwordbussizestorestheinitialbytesorwordsinauxiliary
registers. The CLKB rising edge that writes the fourth byte or the second word
of long word to FIFO2 also stores the entire long word in the FIFO2 memory.
The bytes are arranged in the manner shown in Figure 2.
WhenwritingdatatoFIFO2inbyteorwordformat,theunusedB0-B35inputs
are don't care inputs.