DS1248Y-70 Maxim Integrated Products, DS1248Y-70 Datasheet - Page 5

• Manufacturer Part Number: DS1248Y-70
• Description: Real Time Clock
• Manufacturer: Maxim Integrated Products

Specifications of DS1248Y-70

• Function: Clock, Calendar, NV Timekeeping RAM, Battery Backup
• Rtc Memory Size: 131072 B
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 4.5 V
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Mounting Style: Through Hole
• Rtc Bus Interface: Phantom Clock, Parallel
• Package / Case: eDIP
• Time Format: HH:MM:SS:hh
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

DS1248/DS1248P 1024k NV SRAM with Phantom Clock
PHANTOM CLOCK OPERATION
Communication with the phantom clock is established by pattern recognition on a serial bit stream of
64 bits, which 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, output enable, and write enable. 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 control of the SmartWatch. These 64 write cycles are used only to gain access to the phantom
clock. Therefore, any address to the memory in the socket is acceptable. However, the write cycles
generated to gain access to the phantom clock are also writing data to a location in the mated RAM. The
preferred
way
to
manage
this
requirement
is
to
set
aside
just
one address location in RAM 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 advance and all subsequent write cycles are ignored. If a read cycle occurs at any time during
pattern recognition, the present sequence is aborted and the comparison register pointer is reset. Pattern
recognition continues for a total of 64 write cycles as described above until all the bits in the comparison
register have been matched (Figure 1). With a correct match for 64 bits, the phantom clock is enabled and
data transfer to or from the timekeeping registers can proceed. The next 64 cycles will cause the phantom
clock to either receive or transmit data on DQ0, depending on the level of the OE pin or the WE pin.
Cycles to other locations outside the memory block can be interleaved with CE cycles without
interrupting the pattern recognition sequence or data transfer sequence to the phantom clock.
PHANTOM CLOCK REGISTER INFORMATION
The phantom clock information is contained in eight registers of 8 bits, each of which is sequentially
accessed 1 bit at a time after the 64-bit pattern recognition sequence has been completed. When updating
the phantom clock registers, each register must be handled in groups of 8 bits. Writing and reading
individual bits within a register could produce erroneous results. These read/write registers are defined in
Figure 2.
Data contained in the phantom clock register is in binary-coded decimal format (BCD). Reading and
writing the registers is always accomplished by stepping through all eight registers, starting with bit 0 of
register 0 and ending with bit 7 of register 7.
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