Rainbow Electronics DS1821 - Manual (Page 4)

DS1821
Page 4 of 17
the 1-WIRE BUS SYSTEM section of this datasheet. These communications can include reading and
writing the high and low thermostat trip-point registers (T
H
and T
L
) and the configuration register, and
reading the temperature, counter, and slope accumulator registers. Also in this mode, the microprocessor
can initiate and stop temperature measurements as described in the OPERATION – MEASURING
TEMPERATURE section of this datasheet.
The T
H
and T
L
registers and certain bits (THF, TLF, T/R ¯ , POL and 1SHOT) in the status/configuration
register are stored in nonvolatile EEPROM memory, so they will retain data when the device is powered
down. This allows these registers to be pre-programmed when the DS1821 is to be used as a standalone
thermostat. Writes to these nonvolatile registers can take up to 10ms. To avoid data corruption, no
writes to nonvolatile memory should be initiated while a write to nonvolatile memory is in progress.
Nonvolatile write status can be monitored by reading the NVB bit in the status/configuration register:
NVB = 0 – a write to EEPROM memory is in progress, NVB = 0 – nonvolatile memory is idle.
THERMOSTAT MODE
In thermostat mode (T/R
¯
= 1), the DS1821 can operate as a standalone thermostat that triggers according
to the T
H
and T
L
trip-points programmed while the device was in 1-wire mode. In thermostat mode the
DS1821 powers-up performing continuous temperature conversions, and the DQ pin acts as the
thermostat output. Detailed operation of the thermostat output is provided in the OPERATION –
STANDALONE THERMOSTAT section of this datasheet.
Communications can be re-establish with the DS1821 while it is in thermostat mode by pulling V
DD
to 0V
while the DQ line is held high, and then toggling the DQ line low 16 times as shown in Figure 12. This
temporarily places the DS1821 in 1-wire mode, allowing microprocessor communication with the
DS1821 via the DQ pin. At this time any I/O function can be performed, such as reading/writing the T
H
,
T
L
or configuration registers or reading the temperature register. To return to thermostat mode, the same
procedure can be performed (pulling V
DD
to 0V while the DQ line is held high, and then clocking the DQ
line 16 times) or the power can be cycled. Note that temporarily putting the DS1821 into 1-wire mode
does not change the power-up mode of the device; this can only be changed by rewriting the T/R
¯
bit in
the status/configuration register. Also note that holding both V
DD
and DQ low for more than
approximately 10 seconds will cause the DS1821 to be powered down.
OPERATION – MEASURING TEMPERATURE
DS1821 output temperature data is calibrated in degrees centigrade and is stored in two’s complement
format in the 1-byte (8-bit) temperature register (see Figure 3), which the user can access when the
DS1821 is in 1-wire mode (T/R
¯
= 0 in the status/configuration register). The sign bit (S) indicates if the
temperature is positive or negative; for positive numbers S = 0 and for negative numbers S = 1. Table 2
gives examples of digital output data and the corresponding temperature reading. For Fahrenheit
measurements, a lookup table or conversion routine must be used.
The DS1821 can be configured by the user to take continuous temperature measurements (continuous
conversion mode) or single measurements (one-shot mode). The desired configuration can be achieved
by setting the nonvolatile1SHOT bit in the status/configuration register: 1SHOT = 0 – continuous
conversion mode, 1SHOT = 1 – one-shot mode. Note that the 1SHOT setting only controls the operation
of the device in 1-wire mode; in thermostat mode, continuous temperature conversions are started
automatically at power-up.
In continuous conversion mode, the Start Convert T [EEh] command initiates continuous temperature
conversions, which can be stopped using the Stop Convert T [22h] command. In one-shot mode the Start
Convert T [EEh] command initiates a single temperature conversion after which the DS1821 returns to a
low-power standby state. In this mode, the microprocessor can monitor the DONE bit in the