4144I–AERO–05/04
Figu re 2. Reset (write (read) to Programmable Half Full Flag register)
Wr ite Enab le (W )A w rite cycle is initiated on the falling edge of this input if the Full Flag (FF) is not set.
D ata s et-u p and hold time s mus t be mai ntai ned in the rise time of th e lea ding edge of
the Write Enable (W). Data is stored sequentially in the Ram array, regardless of any
current read operation.
Once half the memory is filled, and during the falling edge of the next write operation,
the Half-Full Flag (HF) w ill be set to low and remain in this s tate until the di fference
between the write and read pointers is less than or equal to half of the total available
memory in the device. The Half-Full Flag (HF) is then reset by the rising edge of the
read operation.
To prevent data overflow, the Full Flag (FF) will go low, inhibiting further write opera-
tions. On completion of a valid read operation, the Full Flag (FF) will go high af ter TRFF,
allowing a valid write to begin. When the FIFO stack is full, the internal write pointer is
blocked f rom W, so that external changes to W will have no effect on the full FIFO stac k.
Read Enable (R)A read cycle is initiated on the falling edge of the Read Enable (R) provided that the
Empty Flag ( EF) is not set. The data is accessed on a first i n/first out basis, not including
any current write operations. After Read Enable (R) goes high, the Data Out puts
(Q0 - Q8 ) will return t o a high imp edance s tate until the next Read operation. When al l
the dat a in the FIFO s tack has bee n read, the E mp ty Flag (E F) will g o low, a ll o w ing t he
“final” read cycle, but inhibiting f urth er read operat ions while the data output s remain in
a high impedance state. Once a valid write operation has been completed, the E mpty
Flag (EF) will go high after tWEF and a valid read may then be initiated. When the FIFO
stack is empty, the internal read pointer is blocked from R, so that external changes to R
will have no effect on the empty FIFO stack.
First Load/Retransm it
(FL/RT) This is a dual-purpose input. In the Depth Expansion Mode, this pin is connected to
ground t o i ndicat e that it is the first loaded (see Operating Mo des ). In the Single Device
Mode , this pin acts as the retransmi t input . Th e Single Device Mod e is initiated by con-
necting the Expansion In (XI) to ground.
The M 672061H ca n be s et to ret ransmi t data when the Retransmit Enable Cont rol (R T)
input is pulsed low. A retransmit operation will set the internal read point to the first loca-
tion and will not affect the write pointer. Read Enable (R) and Write Enable (W) must be
in the high state during retransmit. The retransmit feature is intended for use when a
numb er of writes are equal to or less than the depth of the FIF O has occured since t he
last RS cycle. The retransmit feature is not compatible with the Depth Expansion Mode
and will affect the Half-Full Flag (HF), in accordance with the relative locations of the
read and write pointers.