DS1602 Datasheet, PDF (2/10 Pages) Dallas Semiconductor – Elapsed Time Counter

DS1602
OPERATION
The main elements of the DS1602 are shown in Figure 1. As shown, communications to and from the
elapsed time counter occur over a 3–wire serial port. The port is activated by driving RST to a high state.
With RST at high level, 8 bits are loaded into the protocol shift register providing read/write, register
select, register clear, and oscillator trim information. Each bit is serially input on the rising edge of the
clock input. After the first eight clock cycles have loaded the protocol register with a valid protocol,
additional clocks will output data for a read or input data for a write. V_CC must be present to access the
DS1602. If V_CC < V_BAT, the DS1602 will go into a battery backup mode which disables the serial port to
conserve battery capacity. For battery only operations, the V_BAT pin must be grounded and the V_CC pin
must be connected to the battery. This will keep the DS1602 out of battery backup mode. Battery
powered operation down to 2.5V is possible with reduced speed performance on the serial port.
PROTOCOL REGISTER
The protocol bit definition is shown in Figure 2. Valid protocols and the resulting actions are shown in
Table 1. Each data transfer to the protocol register designates what action is to occur. As defined, the
MSB (bit 7 which is designated ACC) selects the 32–bit continuous counter for access. If ACC is a
logical 1 the continuous counter is selected and the 32 clock cycles that follow the protocol will either
read or write this counter. If the counter is being read, the contents will be latched into a different register
at the end of protocol and the latched contents will be read out on the next 32 clock cycles. This avoids
reading garbled data if the counter is clocked by the oscillator during a read. Similarly, if the counter is to
be written, the data is buffered in a register and all 32 bits are jammed into the counter simultaneously on
the rising edge of the 32^nd clock. The next bit (bit 6 which is designated AVC) selects the 32–bit V_CC
active counter for access. If AVC is a logical 1 this counter is selected and the 32 clock cycles that follow
will either read or write this counter. If both bit 7 and bit 6 are written to a logic high, all clock cycles
beyond the protocol are ignored and bits 5, 4, and 3 are loaded into the oscillator trim register. A value of
binary 3 (011) will give a clock accuracy of ±120 seconds per month at 25°C. Increasing the binary
number towards 7 will cause the real time clock to run faster. Conversely, lowering the binary number
towards 0 will cause the clock to run slower. Binary 000 will stop the oscillator completely. This feature
can be used to conserve battery life during storage. In this mode the I_BAT current is reduced to 100 nA
maximum. In applications where oscillator trimming is not practical or not needed, a default setting of
011 is recommended. Bit 2 of protocol (designated CCC) is used to clear the continuous counter. When
set to logic 1, the continuous counter will reset to 0 when RST is taken low. Bit 1 of protocol (designated
CVC) is used to clear the V_CC active counter. When set to logical 1, the V_CC active counter will reset to 0
when RST is taken low. Both counters can be reset simultaneously by setting CCC and CVC both to a
logical 1. Bit 0 of the protocol (designated RD) determines whether the 32 clocks to follow will write a
counter or read a counter. When RD is set to a logical 0 a write action will follow when RD is set to a
logical 1 a read action will follow. When sending the protocol, 8 bits should always be sent. Sending less
than 8 bits can produce erroneous results. If clearing the counters or trimming the oscillator, the data
transfer can be terminated after the 8–bit protocol is sent. However, when reading or writing the counters,
32 clock cycles should always follow the protocol.
RESET AND CLOCK CONTROL
All data transfers are initiated by driving the RST input high. The RST input has two functions. First,
RST turns on the serial port logic which allows access to the protocol register for the protocol data entry.
Second, the RST signal provides a method of terminating the protocol transfer or the 32–bit counter
transfer. A clock cycle is a sequence of a falling edge followed by a rising edge. For write inputs, data
must be valid during the rising edge of the clock. Data bits are output on the falling edge of the clock
when data is being read. All data transfers terminate if the RST input is transitioned low and the DQ pin
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