DS620 Datasheet, PDF (7/15 Pages) Dallas Semiconductor – Low-Voltage, +-0.5°C Accuracy Digital Thermometer and Thermostat

DS620 Digital Thermometer and Thermostat
THERMOSTAT OPERATION
The PO pin on the DS620 operates as the thermostat output when it is configured as PO_-HIGH or PO_-LOW through the
P01 and P02 bits of the configuration register. In both of these configurations, PO is updated after every
temperature conversion or write to the temperature register, and remains at the updated value until the next
conversion or write completes. PO_-HIGH and PO_-LOW are active-low and are activated and deactivated based on
user-defined upper and lower trip-points. PO_-HIGH is activated when the measured temperature meets or exceeds
the value stored in the upper trip-point register (T_H), and stays active until the temperature meets or falls below the
value stored in the lower trip-point register (T_L) (see Figure 4a). This allows the user to program any amount of
hysteresis into the output response. Similarly, PO_-LOW is activated when the measured temperature meets or goes
below the value stored in the low trip-point register (T_L), and stays active until the temperature meets or exceeds
the value stored in the upper trip-point register (T_H) (see Figure 4b). The T_H register is located in address spaces
A0h (MSB) and A1h (LSB) and the T_L register is located in address spaces A2h (MSB) and A3h (LSB) of the
DS620 memory.
The T_H and T_L registers (see Table 1) contain centigrade temperature values in two’s complement format and are
stored in EEPROM; therefore, they are nonvolatile (NV) and can be programmed prior to installation of the DS620
for use in standalone applications. All bits in the T_H and T_L registers are used in the comparison to the temperature
value in the temperature register for the thermostat operation, regardless of the number of bits used for the
temperature conversions as decided by the R0 and R1 bits in the configuration register. Therefore, to ensure
proper thermostat operation, any bits not used for the temperature measurement should be set to 0 in the T_H and
T_L registers. For example, for 11-bit temperature conversions, bits 3 and 4 in the T_H and T_L register should be set to
0 prior to comparison to the measured temperature. (Bits 0 to 2 are automatically set to 0).
Another thermostat feature is the temperature high and low flags (THF and TLF) in the configuration register.
These bits provide a record of whether the temperature has ever been equal to or greater than T_H or equal to or
less than T_L at anytime since power up or since the bits were last cleared. If the temperature ever meets or
exceeds the T_H register value, the THF bit in the configuration register is set to 1, and if the temperature ever
meets or falls below the T_L value, the TLF bit in the configuration register is set to 1. Once THF or TLF has been
set, it remains set until the power is cycled or it is overwritten with a 0 by the user.
Figure 4. Thermostat Operation
a) PO-high
Logic 1
b) PO-low
Logic 1
Logic 0
T_L
T_H
Te^Tm^ep^m
p
Logic 0
T_L
T_H
T^Te^em^mp
p
STANDALONE THERMOSTAT OPERATION
The DS620 can function as a standalone thermostat, i.e., it can provide thermostat functionality without requiring
communication with a microcontroller. For standalone thermostat operation, the T_H, T_L, and configuration registers
must be programmed to the desired values prior to installation. For standalone operation, the AUTOC bit in the
configuration register must be set to 1 so that measurements begins automatically at power up. This also
configures the A1 pin as an input pin that can trigger a conversion. In addition, PO must be configured as PO_-HIGH
or PO_-LOW. The 1SHOT bit in the configuration register is used to enable the DS620 to perform continuous
conversions at power up (1SHOT = 0) or a single conversion (one-shot) at power up or upon request (1SHOT = 1).
In one-shot mode, one conversion is performed at power-up and then the device enters a low-power standby state
until A1 is toggled high. The A1 pin must be toggled low and back high again to start another conversion.
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