Rainbow Electronics DS1721 - Manual (Page 6)

DS1721
6 of 16
R0, R1 = Thermometer Resolution Bits. Table 3 below defines the resolution of the digital thermometer,
based on the settings of these two bits. There is a direct tradeoff between resolution and conversion time,
as depicted in the DC Electrical Characteristics: Digital Thermometer table. The designer has read/write
access to R0 and R1, and the default state is R0="1" and R1="1" (12-bit conversions).
THERMOMETER RESOLUTION CONFIGURATION Table 3
R1 R0
THERMOMETER
RESOLUTION
MAX
CONVERSION
TIME
0 0 9-BIT 0.15s
0 1 10-BIT 0.3s
1 0 11-BIT 0.6s
1 1 12-BIT 1.2s
X = Undefined. These bits are used internally by the DS1721.
DONE = Temperature Conversion Status Bit. "1" = conversion complete and "0" = conversion in
progress. The DONE bit is read-only, and the power-up state is "1". In the continuous conversion mode,
DONE = "0".
Thermostat Setpoints Programming
The thermostat registers (TH and TL) define the setpoints for operation of the TCOM output. The
respective register can be accessed over the 2-wire bus via the Access TH (A1h) or Access TL (A2h)
commands. Reading from or writing to the respective register is controlled by the state of the R/
W bit in
the 2-wire control byte (See “2-Wire Serial Data Bus” section).
The format of the TH and TL registers is a 12-bit 2’s complement representation of the temperature in °C.
The user can program the number of bits (9, 10, 11, or 12) for each TH and TL that correspond to the
thermometer resolution configuration chosen. If the 9-bit mode is chosen, for example, the 3 least
significant bits of TH and TL will be ignored by the thermostat comparator. The format for both TH and
TL is shown in Figure 4. The power-up default of TH is 80°C and that for TL is 75°C.
TEMPERATURE/DATA RELATIONSHIPS Figure 4
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb (UNIT = °C) LSb
2
-1
2
-2
2
-3
2
-4
0 0 0 0 LSB
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+80°C 0101 0000 0000 0000 5000h
+75°C 0100 1011 0000 0000 4B00h
+10.125°C 0000 1010 0010 0000 0A20h
+0.5°C 0000 0000 1000 0000 0080h
+0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1000 0000 FF80h
-10.125°C 1111 0101 1110 0000 F5E0h
-25.0625°C 1110 0110 1111 0000 E6F0h
-55°C 1100 1001 0000 0000 C900h