Rainbow Electronics DS17887 - Manual (Page 19)

DS17885/DS17887
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During interval 5, the system is fully powered down. Battery backup of the clock calendar and NV RAM
is in effect and
IRQ is tri-stated, and monitoring of wake-up and kickstart takes place. If PRS = 1, PWR
stays active; otherwise, if PRS = 0 PWR is tri-stated.
RAM Clear
The DS17885/DS17887 provides a RAM clear function for the 114 bytes of user RAM. When enabled,
this function can be performed regardless of the condition of the V
CC
pin.
The RAM clear function is enabled or disabled through the RAM clear-enable bit (RCE; bank 1, register
04BH). When this bit is set to a logic 1, the 114 bytes of user RAM is cleared (all bits set to 1) when an
active low transition is sensed on the
RCLR pin. This action has no affect on either the clock/calendar
settings or upon the contents of the extended RAM. The RAM clear flag (RF, bank 1, register 04AH) is
set when the RAM clear operation has been completed. If V
CC
is present at the time of the RAM clear and
RIE = 1, the IRQ line is also be driven low upon completion. The interrupt condition can be cleared by
writing a 0 to the RF bit. The IRQ line then returns to its inactive high level provided there are no other
pending interrupts. Once the RCLR pin is activated, all read/write accesses are locked out for a minimum
recover time, specified as t
REC
in Electrical Characteristics.
When RCE is cleared to 0, the RAM clear function is disabled. The state of the RCLR pin has no affect
on the contents of the user RAM, and transitions on the RCLR pin have no affect on RF.
4k x 8 Extended RAM
The DS17885/DS17887 provides 4k x 8 of on-chip SRAM that is controlled as nonvolatile storage
sustained from a lithium battery. On power-up, the RAM is taken out of write-protect status by the
internal power-OK signal (POK) generated from the write-protect circuitry. The on-chip 4k x 8 NV
SRAM is accessed through the eight multiplexed address/data lines AD7 to AD0. Access to the SRAM is
controlled by three on-chip latch registers. Two registers are used to hold the SRAM address, and the
other register is used to hold read/write data. The SRAM address space is from 00h to 0FFFh.
Access to the extended 4k x 8 RAM is controlled by three of the Dallas registers shown in Figure 4. The
Dallas registers in bank 1 must first be selected by setting the DV0 bit in register A to a logic 1. The 12-
bit address of the RAM location to be accessed must be loaded into the extended RAM address registers
located at 50h and 51h. The least significant address byte should be written to location 50h, and the most
significant 4-bits (right-justified) should be loaded in location 51h. Data in the addressed location can be
read by performing a read operation from location 53h, or written to by performing a write operation to
location 53h. Data in any addressed location can be read or written repeatedly without changing the
address in location 50h and 51h.
To read or write consecutive extended RAM locations, a burst mode feature can be enabled to increment
the extended RAM address. To enable the burst mode feature, set the BME bit in the extended control
register 4Ah, to a logic 1. With burst mode enabled, write the extended RAM starting address location to
registers 50h and 51h. Then read or write the extended RAM data from/to register 53h. The extended
RAM address locations are automatically incremented on the rising edge of
RD or WR only when
register 53h is being accessed. Refer to the Burst Mode Timing Waveform.