Rainbow Electronics DS1254 - Manual (Page 3)

DS1254
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RAM READ MODE
The DS1254 executes a read cycle whenever WE is inactive (high) and CE is active (low). The unique
address specified by the 21 address inputs (A0–A20) defines which of the 2MB of data is to be accessed.
Valid data will be available to the eight data-output drivers within t
ACC
(Access Time) after the last
address input is stable, providing that CE and OE access times and states are also satisfied. If OE and CE
access times are not satisfied, then data access must be measured from the later occurring signal ( CE or
OE ) and the limiting parameter is either t
CO
for CE or t
OE
for OE rather than address access.
RAM WRITE MODE
The DS1254 is in the write mode whenever WE and CE are in their active (low) state after address inputs
are stable. The later occurring falling edge of CE or WE will determine the start of the write cycle. The
write cycle is terminated by the earlier rising edge of CE or WE . All address inputs must be kept valid
throughout the write cycle. WE must return to the high state for a minimum recovery time (t
WR
) before
another cycle can be initiated. The OE control signal should be kept inactive (high) during write cycles to
avoid bus contention. However, if the output bus has been enabled ( CE and OE active), then WE will
disable the outputs in t
ODW
from its falling edge.
DATA RETENTION MODE
The device is fully accessible and data can be written and read only when V
CC
is greater than V
PF
.
However, when V
CC
falls below the power- fail point, V
PF
(point at which write protection occurs), the
internal clock registers and SRAM are blocked from any access. When V
CC
falls below V
BAT
, device
power is switched from the V
CC
to V
BAT
. RTC operation and SRAM data are maintained from the battery
until V
CC
is returned to nominal levels. All signals must be powered down when V
CC
is powered down.
PHANTOM CLOCK OPERATION
Communication with the phantom clock is established by pattern recognition on a serial bit stream of
64 bits that must be matched by executing 64 consecutive write cycles containing the proper data on
DQ0. All accesses that occur prior to recognition of the 64-bit pattern are directed to memory.
After recognition is established, the next 64 read or write cycles either extract or update data in the
phantom clock, and memory access is inhibited.
Data transfer to and from the timekeeping function is accomplished with a serial bit stream under control
of chip enable ( CE ), output enable ( OE ), and write enable ( WE ). Initially, a read cycle to any memory
location using the CE and OE control of the phantom clock starts the pattern recognition sequence by
moving a pointer to the first bit of the 64-bit comparison register. Next, 64 consecutive write cycles are
executed using the CE and WE signals of the device. These 64 write cycles are used only to gain access to
the phantom clock. Therefore, any address within the first 512kB of memory, (00h to 7FFFFh) is
acceptable. However, the write cycles generated to gain access to the phantom clock are also writing data
to a location in the memory. The preferred way to manage this requirement is to set aside just one address
location in memory as a phantom clock scratch pad. When the first write cycle is executed, it is compared
to bit 0 of the 64-bit comparison register. If a match is found, the pointer increments to the next location
of the comparison register and awaits the next write cycle. If a match is not found, the pointer does not