Bidirectional Mode
Applications which require data buffering between two systems (each system being
capable of Read and Write operations) can be created by coupling M67204H as shown
in Figure 6. Care must be taken to ensure that the appropriate flag is monitored by each
system (i.e. FF is monitored on the device on which W is in use; EF is monitored on the device
on which R is in use). Both Depth Expansion and Width Expansion may be used in this mode.
Two types of flow-through modes are permitted: a read flow-through and a write flow-
through mode. In the read flow-through mode (Figure 17) the FIFO stack allows a single
word to be read after one word has been written to an empty FIFO stack. The data is
enabled on the bus at (tWEF + tA) ns after the leading edge of W which is known as the
first write edge and remains on the bus until the R line is raised from low to high, after which the
bus will go into a three-state mode after tRHZ ns. The EF line will show a pulse indicating tem-
porary reset and then will be set. In the interval in which R is low, more words may be written to
the FIFO stack (the subsequent writes after the first write edge will reset the Empty Flag); how-
ever, the same word (written on the first write edge) presented to the output bus as the read
pointer will not be incremented if R is low. On toggling R, the remaining words written to the
FIFO will appear on the output bus in accordance with the read cycle timings.
In the write flow-through mode (Figure 18), the FIFO stack allows a single word of data
to be written immediately after a single word of data has been read from a full FIFO
stack. The R line causes the FF to be reset, but the W line, being low, causes it to be set again
in anticipation of a new data word. The new word is loaded into the FIFO stack on the leading
edge of W. The W line must be toggled when FF is not set in order to write new data into the
FIFO stack and to increment the write pointer.
Figure 4.
Block Diagram of 12288 bits
×
9 FIFO Memory (Depth Expansion)
Data Flow – Through
Modes
8
M67204H
4141J–AERO–04/07
ATMEL [ ATMEL CORPORATION ]
