Rainbow Electronics DS1642 - Manual (Page 2)

DS1642
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prevents time loss as the timekeeping countdown continues unabated by access to time register data. The
DS1642 also contains its own power-fail circuitry which deselects the device when the V
CC
supply is in
an out-of-tolerance condition. This feature prevents loss of data from unpredictable system operation
brought on by low V
CC
as errant access and update cycles are avoided.
CLOCK OPERATIONS-READING THE CLOCK
While the double-buffered register structure reduces the chance of reading incorrect data, internal updates
to the DS1642 clock registers should be halted before clock data is read to prevent reading of data in
transition. However, halting the internal clock register updating process does not affect clock accuracy.
Updating is halted when a 1 is written into the read bit, the 7th most significant bit in the control register.
As long as a 1 remains in that position, updating is halted. After a halt is issued, the registers reflect the
count, that is day, date, and time that was current at the moment the halt command was issued. However,
the internal clock registers of the double-buffered system continue to update so that the clock accuracy is
not affected by the access of data. All of the DS1642 registers are updated simultaneously after the clock
status is reset. Updating is within a second after the read bit is written to 0.
DS1642 BLOCK DIAGRAM Figure 1
DS1642 TRUTH TABLE Table 1
V
CC CE OE WE
MODE DQ POWER
V
IH
X X DESELECT HIGH Z STANDBY
V
IL
X V
IL
WRITE DATA IN ACTIVE
V
IL
V
IL
V
IH
READ DATA OUT ACTIVE
5 VOLTS ± 10%
V
IL
V
IH
V
IH
READ HIGH Z ACTIVE
<4.5 VOLTS >V
BAT
X X X DESELECT HIGH Z CMOS STANDBY
BAT
X X X DESELECT HIGH Z DATA RETENTION
MODE