Inter-Integrated Circuit (I2C) Interface - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 567

taking temperature measurements at power-up; this allows it to function as a stand-alone ther-
mostat. The DS1631A conforms to the I 2 C bus specification.

11.7.3 DS1631A Registers
Four registers are involved in the operation of the DS1631A: Config, T H , T L , and Tempera-
ture registers. The SRAM-based Temperature register is a 2-byte register that holds the converted
temperature value in two's complement format. The converted temperature is stored in the up-
per bits of the Temperature register with lower bits filled with 0s. When the most significant bit
of this register is 1, the temperature is negative.
Both the T H and T L are EEPROM-based 2-byte registers. T H holds the upper alarm tempera-
ture value in two's complement format. Whenever the converted temperature value is equal
to or higher than the value in T H , the T OUT signal will be asserted. Once asserted, T OUT can be
de-asserted only when the converted temperature is lower than the value in the T L register. The
value in the T L register is also represented in two's complement format.
The contents of the Config register are shown in Figure 11.33. The lower 2 bits of the con-
figuration register are EEPROM-based, whereas the upper 6 bits are SRAM-based. The meaning
of each bit is well defined. The Config register allows the designer to program various DS1631A

7 5 4 3 2 1 0

6

power-up
value=100011XX

DONE THF TLF NVB R1 R0 POL* 1SHOT*

*NV (EEPROM)

Done: temperature conversion done (read-only)
0 = temperature conversion is in progress.
1 = temperature conversion is complete. Will be cleared when the Temperature
register is read.
THF: temperature high flag (read/write)
0 = the measured temperature has not exceeded the value in T H register.
1 = the measured temperature has exceeded the value in T H register. THF
remains at 1 until it is overwritten with a 0 by the user, the power is
recycled, or a software POR command is issued.
TLF: temperature low flag (read/write)
0 = the measured temperature has not been lower than the value in T L register.
1 = at some point after power-up, the measured temperature is lower than the
value stored in the T L register. TLF remains at 1 until it is overwritten with a
0 by the user, the power is recycled, or a software POR command is issued.
NVB: nonvolatile memory busy (read only)
0 = NV memory is not busy.
1 = A write to EEPROM memory is in progress.
R1:R0 : resolution bits (read/write)
00 = 9-bit resolution (conversion time is 93.75 ms).
01 = 10-bit resolution (conversion time is 187.5 ms).
10 = 11-bit resolution (conversion time is 375 ms).
11 = 12-bit resolution (conversion time is 750 ms).
POL: T OUT polarity (read/write)
0 = T OUT active low.
1 = T OUT active high.
1SHOT: conversion mode (read/write)
0 = continuous conversion mode. The Start Convert T command initiates
continuous temperature conversions.
1 = one-shot mode. The Start Convert T command initiates a single temperature
conversion and then the device enters a low-power standby mode.

Figure 11.33 ■ DS1631A Config register