DS1286 Datasheet, PDF (2/18 Pages) Dallas Semiconductor – Watchdog Timekeeper

DS1284/DS1286
OPERATION—READ REGISTERS
The DS1284/DS1286 execute a read cycle whenever WE (write enable) is inactive (high) and CE (chip
enable) and OE (output enable) are active (low). The unique address specified by the six address inputs
(A0–A5) defines which of the 64 registers is to be accessed. Valid data is available to the eight data
output drivers within t_ACC (access time) after the last address input signal is stable, provided that CE and
OE access times are also satisfied. If OE and CE access times are not satisfied, then data access must be
measured from the latter occurring signal (CE or OE) and the limiting parameter is either t_CO for CE or t_OE
for OE rather than address access.
OPERATION—WRITE REGISTERS
The DS1284/DS1286 are in the write mode whenever the WE and CE signals are in the active-low state
after the address inputs are stable. The latter occurring falling edge of CE or WE determines 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
state (t_WR) before another cycle can be initiated. Data must be valid on the data bus with sufficient data
setup (t_DS) and data hold time (t_DH) with respect to the earlier rising edge of CE or WE. 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
The watchdog timekeeper provides full functional capability when V_CC is greater than V_TP. Data is
maintained in the absence of V_CC without any additional support circuitry. The DS1284/DS1286
constantly monitor V_CC. Should the supply voltage decay, the watchdog timekeeper automatically write
protects itself, and all inputs to the registers become “don’t care.” Both INTA and INTB (INTB) are
open-drain outputs. The two interrupts and the internal clock continue to run regardless of the level of
V_CC. However, it is important to ensure that the pullup resistors used with the interrupt pins are never
pulled up to a value greater than V_CC + 0.3V. As V_CC falls below the battery voltage, a power-switching
circuit turns on the lithium energy source to maintain the clock and timer data functionality. Also ensure
that during this time (battery-backup mode), the voltage present at INTA and INTB (INTB) never
exceeds the battery voltage. If the active-high mode is selected for INTB (INTB), this pin only goes high
in the presence of V_CC. During power-up, when V_CC rises above approximately 3.0V, the power-switching
circuit connects external V_CC and disconnects the V_BAT energy source. Normal operation can resume after
V_CC exceeds V_TP for t_REC.
WATCHDOG TIMEKEEPER REGISTERS
The watchdog timekeeper has 64 8-bits-wide registers that contain all the timekeeping, alarm, watchdog,
control, and data information. The clock, calendar, alarm, and watchdog registers are memory locations
that contain external (user-accessible) and internal copies of the data. The external copies are independent
of internal functions, except that they are updated periodically by the simultaneous transfer of the
incremented internal copy (see Figure 1). The command register bits are affected by both internal and
external functions. This register is discussed later. The 50 bytes of RAM registers can only be accessed
from the external address and data bus. Registers 0, 1, 2, 4, 6, 8, 9, and A contain time-of-day and date
information (see Figure 2). Time-of-day information is stored in binary-coded decimal (BCD). Registers
3, 5, and 7 contain the time-of-day alarm information. Time-of-day alarm information is stored in BCD.
Register B is the command register and information in this register is binary. Registers C and D are the
watchdog alarm registers and information stored in these two registers is in BCD. Registers E to 3F are
user bytes and can be used to contain data at the user’s discretion.
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