ADM1027
FUNCTIONAL DESCRIPTION
Internal Registers of the ADM1027
General Description
A brief description of the ADM1027’s principal internal regis-
ters follows. More detailed information on the function of each
register is given in Tables IV to XXXVI.
The ADM1027 is a complete systems monitor and multiple fan
controller for any system requiring monitoring and cooling. The
device communicates with the system via a serial system
management bus. The serial bus controller has an optional
address line for device selection (Pin 14), a serial data line
for reading and writing addresses and data (Pin 1), and an input
line for the serial clock (Pin 2). All control and programming
functions of the ADM1027 are performed over the serial bus. In
addition, one of the pins can be reconfigured as an SMBALERT
output to indicate out-of-limit conditions.
Configuration Registers
Provide control and configuration of the ADM1027, including
alternate pinout functionality.
Address Pointer Register
Contains the address that selects one of the other internal registers.
When writing to the ADM1027, the first byte of data is always a
register address, which is written to the Address Pointer Register.
Status Registers
Measurement Inputs
Provide the status of each limit comparison and are used to
signal out-of-limit conditions on the temperature, voltage, or
fan speed channels. If Pin 10 is configured as SMBALERT,
then this pin will assert low whenever a status bit gets set.
The device has six measurement inputs, four for voltage and
two for temperature. It can also measure its own supply voltage
and can measure ambient temperature with its on-chip tempera-
ture sensor.
Interrupt Mask Registers
Pins 20 to 23 are analog inputs with on-chip attenuators,
configured to monitor 5 V, 12 V, 2.5 V, and the processor core
voltage (2.25 V input), respectively.
Allow each interrupt status event to be masked when Pin 10 is
configured as an SMBALERT output. This affects only the
SMBALERT output and not the bits in the status register.
Power is supplied to the chip via Pin 4, which the system also
uses to monitor VCC. In PCs, this pin is normally connected to a
3.3 V standby supply. This pin can, however, be connected to a
5 V supply and monitor it without overranging.
VID Register
The status of the VID0 to VID4 pins of the processor can be
read from this register.
Value and Limit Registers
Remote temperature sensing is provided by the D1+/– and
D2+/– inputs, to which diode-connected, external temperature-
sensing transistors such as a 2N3906 or CPU thermal diode
may be connected.
The results of analog voltage inputs, temperature, and fan speed
measurements are stored in these registers, along with their limit
values.
Offset Registers
The ADC also accepts input from an on-chip band gap tem-
perature sensor that monitors system ambient temperature.
Allow each temperature channel reading to be offset by a twos
complement value written to these registers.
Sequential Measurement
TMIN Registers
When the ADM1027 monitoring sequence is started, it cycles
sequentially through the measurement of analog inputs and the
temperature sensors. Measured values from these inputs are
stored in value registers. These can be read out over the serial
bus, or can be compared with programmed limits stored in the
limit registers. The results of out-of-limit comparisons are stored
in the status registers, which can be read over the serial bus to
flag out-of-limit conditions.
Program the starting temperature for each fan under automatic
fan speed control.
TRANGE Registers
Program the temperature-to-fan speed control slope in automatic
Fan Speed Control Mode for each PWM output.
Enhance Acoustics Registers
Allow each PWM output controlling fan to be tweaked to enhance
the system’s acoustics.
Processor Voltage ID
Five digital inputs (VID0 to VID4 — Pins 5 to 8 and 19) read
the processor Voltage ID code and store it in the VID register,
from which it can be read out by the management system over
the serial bus. The VID code monitoring function is compatible
with both VRM9.x and future VRM10 solutions. The VID code
monitoring function is compatible with VRM9.x.
ADM1027 Address Selection
Pin 13 is the dual function PWM3/ADDRESS ENABLE pin.
If Pin 13 is pulled low on power-up, the ADM1027 will read the
state of Pin 14 (TACH4/ADDRESS SELECT pin) to determine
the ADM1027 slave address. If Pin 13 is high on power-up, then
the ADM1027 will default to SMBus slave address 0x5C. This
function is described later in more detail.
Rev. 3 | Page 6 of 56 | www.onsemi.com
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