DS1821C+【PDF 5/19 页】IC THERMOMETER/STAT PROG 3-TO92

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Sheet目录39

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DS1821

5 of 19

Nonvolatile write status can be monitored by reading the NVB bit in the status/configuration register:

NVB = 1 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

and T

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-established with the DS1821 while it is in thermostat mode by pulling V

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

or configuration registers or reading the temperature register. To return to thermostat mode, the same

procedure can be performed (pulling V

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

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 twos 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

configuration register to determine when the conversion is complete: DONE = 0 conversion in

progress, DONE = 1 conversion complete. The DONE bit does not provide conversion status in

continuous conversion mode since measurements are constantly in progress (i.e., DONE will always be

0).

TEMPERATURE, T

and T

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

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