DS1720
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For typical thermostat operation, the DS1720 will operate in continuous mode. However, for applications
where only one reading is needed at certain times, and to conserve power, the one–shot mode may be
used. Note that the thermostat outputs (T
HIGH
, T
LOW
, T
COM
) will remain in the state they were in after the
last valid temperature conversion cycle when operating in one–shot mode.
OPERATION IN STAND–ALONE MODE
In applications where the DS1720 is used as a simple thermostat, no CPU is required. Since the
temperature limits are nonvolatile, the DS1720 can be programmed prior to insertion in the system. In
order to facilitate operation without a CPU, the CLK/CONV pin (pin 2) can be used to initiate
conversions. Note that the CPU bit must be set to 0 in the configuration register to use this mode of
operation. Whether CPU=0 or 1, the 3–wire port is active. Setting CPU=1 disables the stand–alone mode.
To use the CLK/CONV pin to initiate conversions, RST must be low and CLK/CONV must be high. If
CLK/CONV is driven low and then brought high in less than 10 ms, one temperature conversion will be
performed and then the DS1720 will return to an idle state. If CLK/CONV is driven low and remains low,
continuous con-versions will take place until CLK/CONV is brought high again. With the CPU bit set to
0, the CLK/CONV will override the 1–shot bit if it is equal to 1. This means that even if the part is set for
one–shot mode, driving CLK/CONV low will initiate conversions.
3–WIRE COMMUNICATIONS
The 3–wire bus is comprised of three signals. These are the RST (reset) signal, the CLK (clock) signal,
and the DQ (data) signal. All data transfers are initiated by driving the RST input high. Driving the RST
input low terminates communication. (See Figures 4 and 5). A clock cycle is a sequence of a falling edge
followed by a rising edge. For data inputs, the data must be valid during the rising edge of a clock cycle.
Data bits are output on the falling edge of the clock, and remain valid through the rising edge.
When reading data from the DS1720, the DQ pin goes to a high impedance state while the clock is high.
Taking RST low will terminate any communication and cause the DQ pin to go to a high impedance
state.
Data over the 3–wire interface is communicated LSB first. The command set for the 3–wire interface as
shown in Table 3 is as follows; only these protocols should be written to the DS1720, as writing other
protocols to the device may result in permanent damage to the part.
Read Temperature [AAh]
This command reads the contents of the register which contains the last temperature conversion result.
The next nine clock cycles will output the contents of this register.
Write T
H
[01h]
This command writes to the T
H
(HIGH TEMPERATURE) register. After issuing this command, the next
nine clock cycles clock in the 9–bit temperature limit which will set the threshold for operation of the
T
HIGH
output.
Write T
L
[02h]
This command writes to the T
L
(LOW TEMPERATURE) register. After issuing this command, the next
nine clock cycles clock in the 9–bit temperature limit which will set the threshold for operation of the
T
LOW
output.
