ADM1021A
Rev. 7 | Page 12 of 19 | www.onsemi.com
LIMIT REGISTERS
The ADM1021A has four limit registers to store local and
remote and high and low temperature limits. These registers
can be written to and read back over the SMBus. The high limit
registers perform a > comparison, while the low limit registers
perform a < comparison. For example, if the high limit register
is programmed as a limit of 80° C, measuring 81°C results in an
alarm condition. Even though the temperature measurement
range is from 0° to 127°C, it is possible to program the limit
register with negative values. This is for backwards compati-
bility with the ADM1021.
OFFSET REGISTER
An offset register is provided at Address 0x11. This allows the
user to remove errors from the measured remote temperature.
These errors can be introduced by clock noise and PCB track
resistance. See Table 9 for an example of offset values.
The offset value is stored as an 8-bit, twos complement value.
The value of the offset is negative if the MSB of Register 0x11 is
1, and is positive if the MSB of Register 0x11 is 0. This value is
added to the remote temperature. The offset register defaults to
0 at power-up. The offset register range is −128°C to +127°C.
Table 9. Offset Values
Remote Remote
Offset Temperature Temperature
Register Offset (With (Without
(0x11) Value Offset) Offset)
1111 1100 −4°C 14°C 18°C
1111 1111 −1°C 17°C 18°C
0000 0000 0°C 18°C 18°C
0000 0001 +1°C 19°C 18°C
0000 0100 +4°C 22°C 18°C
ONE-SHOT REGISTER
The one-shot register is used to initiate a single conversion and
comparison cycle when the ADM1021A is in standby mode,
after which the device returns to standby. This is not a data
register as such, and it is the write operation that causes the
one-shot conversion. The data written to this address is
irrelevant and is not stored.
SERIAL BUS INTERFACE
Control of the ADM1021A is carried out via the serial bus. The
ADM1021A is connected to this bus as a slave device, under the
control of a master device. Note that the SMBus and SCL pins
are three-stated when the ADM1021A is powered down and
will not pull down the SMBus.
ADDRESS PINS
In general, every SMBus device has a 7-bit device address
(except for some devices that have extended 10-bit addresses).
When the master device sends a device address over the bus,
the slave device with that address responds.
The ADM1021A has two address pins, ADD0 and ADD1, to
allow selection of the device address so that several
ADM1021As can be used on the same bus, and/or to avoid
conflict with other devices. Although only two address pins
are provided, these are three-state and can be grounded, left
unconnected, or tied to V
DD
so that a total of nine different
addresses are possible, as shown in Table 10.
It should be noted that the state of the address pins is only
sampled at power-up, so changing them after power-up has
no effect.
Table 10. Device Addresses
ADD0
1
ADD1
1
Device Address
0 0 0011 000
0 NC 0011 001
0 1 0011 010
NC 0 0101 001
NC NC 0101 010
NC 1 0101 011
1 0 1001 100
1 NC 1001 101
1 1 1001 110
1
ADD0, ADD1 sampled at power-up only.
The serial bus protocol operates as follows:
1.
The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial
data line SDATA, while the serial clock line SCLK remains
high. This indicates that an address/data stream will
follow. All slave peripherals connected to the serial bus
respond to the START condition and shift in the next
eight bits, consisting of a 7-bit address (MSB first) plus an
R/
W
bit, which determines the direction of the data
transfer, that is, whether data will be written to or read
from the slave device.
The peripheral whose address corresponds to the
transmitted address responds by pulling the data line low
during the low period before the ninth clock pulse, known
as the Acknowledge Bit. All other devices on the bus now
remain idle while the selected device waits for data to be
read from or written to it. If the R/
W
bit is a 0, the master
writes to the slave device. If the R/
W
bit is a 1, the master
reads from the slave device.
