DS1217A Datasheet, PDF (2/8 Pages) Dallas Semiconductor – Nonvolatile Read/Write Cartridge

DS1217A
READ MODE
The DS1217A executes a read cycle whenever WE
(write enable) is inactive (high) and CE (cartridge en-
able) is active (low). The unique address specified by
the 15 address inputs (A0-A14) defines which of the
32,768 bytes of data is to be accessed. Valid data will be
available to the eight data I/O pins within t_ACC (access
time) after the last address input signal is stable, provid-
ing that CE and OE (output enable) access times are
also satisfied. If OE and CE times are not satisfied , then
data access must be measured from the latter occurring
signal (CE or OE); the limiting parameter is either t_CO for
CE or t_OE for OE rather than address access. Read
cycles can only occur when V_CC is greater than 4.5
volts. When V_CC is less than 4.5 volts, the memory is
inhibited and all accesses are ignored.
WRITE MODE
The DS1217A is in the write mode whenever both the
WE and CE signals are in the active (low) state after ad-
dress inputs are stable. The last falling edge to occur of
either CE or WE will determine the start of the write
cycle. The write cycle is terminated by the first rising
edge of either 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. Write cycles
can only occur when V_CC is greater than 4.5 volts.
When V_CC is less than 4.5 volts, the memory is write
protected.
DATA RETENTION MODE
The Nonvolatile Cartridge provides full functional capa-
bility for V_CC greater than 4.5 volts and guarantees write
protection for V_CC less than 4.5 volts. Data is main-
tained in the absence of V_CC without any additional sup-
port circuitry. The DS1217A constantly monitors V_CC.
Should the supply voltage decay, the RAM is automati-
cally write protected below 4.5 volts. As V_CC falls below
approximately 3.0 volts, the power switching circuit con-
nects a lithium energy source to RAM. To retain data
during power-up, when V_CC rises above approximately
3.0 volts, the power switching circuit connects the exter-
nal V_CC to the RAM and disconnects the lithium energy
source. Normal RAM operation can resume after V_CC
exceeds 4.5 volts.
The DS1217A checks battery status to warn of potential
data loss. Each time that V_CC power is restored to the
cartridge, the battery voltage is checked with a precision
comparator. If the battery supply is less than 2.0 volts,
the second memory cycle is inhibited. Battery status
can, therefore, be determined by performing a read
cycle after power-up to any location in memory, record-
ing that memory location content. A subsequent write
cycle can then be executed to the same memory loca-
tion, altering data. If the next read cycle fails to verify the
written data, the contents of the memory are question-
able.
In many applications, data integrity is paramount. For
this reason, the cartridge provides battery redundancy.
The DS1217A features an internal isolation switch that
provides for the connection of two batteries. During bat-
tery backup time, the battery with the highest voltage is
selected for use. If one battery fails, the other will auto-
matically take over. The switch between batteries is
transparent to the user. A battery status warning will oc-
cur if both batteries are less than 2.0 volts.
REMOTE CONNECTION VIA A RIBBON
CABLE
Existing systems that contain 28-pin bytewide sockets
can be retrofitted using a 28-pin DIP plug. The DIP plug,
AMP Part Number 746616-2, can be inserted into the
28-pin site after the memory is removed. Connection to
the cartridge is accomplished via a 28-pin ribbon cable
connected to a 30-contact card edge connector, AMP
Part Number 499188-4. The 28-pin ribbon cable must
be right-justified such that positions A1 and B1 are left
disconnected. For applications where the cartridge is
installed or removed with power applied, both ground
contacts (A1 and B15) on the card edge connector
should be grounded to further enhance data integrity.
Access time push-out may occur as the distance be-
tween the cartridge and driving circuitry is increased.
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