DS1609
020499 2/7
OPERATION – READ CYCLE
The main elements of the dual port RAM are shown in
Figure 1.
A read cycle to either port begins by placing an address
on the multiplexed bus pins AD0 – AD7. The port enable
control (CE
) is then transitioned low. This control signal
causes address to be latched internally. Addresses can
be removed from the bus provided address hold time is
met. Next, the output enable control (OE
) is transitioned
low, which begins the data access portion of the read
cycle. With both CE and OE active low, data will appear
valid after the output enable access time t
OEA
. Data will
remain valid as long as both port enable and output en-
able remains low. A read cycle is terminated with the
first occurring rising edge of either CE or OE. The ad-
dress/data bus will return to a high impedance state af-
ter time t
CEZ
or t
OEZ
as referenced to the first occurring
rising edge. WE must remain high during read cycles.
OPERATION – WRITE CYCLE
A write cycle to either port begins by placing an address
on the multiplexed bus pins AD0 – AD7. The port enable
control (CE
) is then transitioned low. This control signal
causes address to be latched internally. As with a read
cycle, the address can be removed from the bus pro-
vided address hold time is met. Next the write enable
control signal (WE
) is transitioned low which begins the
write data portion of the write cycle. With both CE and
WE
active low the data to be written to the selected
memory location is placed on the multiplexed bus. Pro-
vided that data setup (t
DS
) and data hold (t
DH
) times are
met, data is written into the memory and the write cycle
is terminated on the first occurring rising edge of either
CE
or WE. Data can be removed from the bus as soon
as the write cycle is terminated. OE
must remain high
during write cycles.
ARBITRATION
The DS1609 dual port RAM has a special cell design
that allows for simultaneous accesses from two ports
(see Figure 2). Because of this cell design, no arbitra-
tion is required for read cycles occurring at the same in-
stant. However, an argument for arbitration can be
made for reading and writing the cell at the exact same
instant or for writing from both ports at the same instant.
A simple way to assure that read/write conflicts don’t oc-
cur is to perform redundant read cycles. Write/write ar-
bitration needs can be avoided by assigning groups of
addresses for write operation to one port only. Groups
of data can be assigned check sum bytes which would
guarantee correct transmission. A software arbitration
system using a “mail box” to pass status information can
also be employed. Each port could be assigned a
unique byte for writing status information which the oth-
er port would read. The status information could tell the
reading port if any activity is in progress and indicate
when activity is going to occur.
