ADT7476
Rev. B | Page 28 of 72
Enabling the SMBALERT Interrupt Output
The SMBALERT interrupt function is disabled by default.
Pin 10 or Pin 14 can be reconfigured as an SMBALERT output
to signal out-of-limit conditions.
Table 15. Configuring Pin 10 as SMBALERT Output
Register Bit
Setting
Configuration Register 3 (0x78)
[1] Pin 10 = SMBALERT
[0] Pin 10 = PWM2
Assigning THERM Functionality to a Pin
Pin 14 on the ADT7476 has four possible functions: SMBALERT,
THERM, GPIO6, and TACH4. The user chooses the required
functionality by setting Bit 0 and Bit 1 of Configuration Register 4
(0x7D).
If THERM is enabled (Configuration Register 3 (0x78), Bit 1),
the following occurs:
•
Pin 22 becomes THERM.
•
If Pin 14 is configured as THERM (Bit 0 and Bit 1 of
Configuration Register 4 at Address 0x7D), THERM is
enabled on this pin.
If THERM is not enabled, the following occurs:
•
Pin 22 becomes a 2.5 V measurement input.
•
If Pin 14 is configured as THERM, then THERM is
disabled on this pin.
Table 16. Configuring Pin 14
Bit 0
Bit 1
Function
0 0 TACH4
0 1 THERM
1 0 SMBALERT
1 1 GPIO6
THERM as an Input
When THERM is configured as an input, the user can time
assertions on the THERM pin. This can be useful for connect-
ing to the PROCHOT output of a CPU to gauge system
performance.
The user can also set up the ADT7476 so that, when the
THERM pin is driven low externally, the fans run at 100%. The
fans run at 100% for the duration of the time that the THERM
pin is pulled low. This is done by setting the BOOST bit (Bit 2)
in Configuration Register 3 (0x78) to 1. This works only if the
fan is already running, for example, in manual mode when the
current duty cycle is above Register 0x00 or in automatic mode
when the temperature is above T
MIN
.
If the temperature is below T
MIN
or if the duty cycle in manual
mode is set to Register 0x00, pulling the THERM low externally
has no effect. See Figure 31 for more information.
05
38
2-
0
32
THERM
T
MIN
THERM ASSERTED TO LOW AS AN INPUT:
FANS DO NOT GO TO 100% BECAUSE
TEMPERATURE IS BELOW T
MIN
.
THERM ASSERTED TO LOW AS AN INPUT:
FANS DO NOT GO TO 100% BECAUSE
TEMPERATURE IS ABOVE T
MIN
AND FANS
ARE ALREADY RUNNING.
Figure 31. Asserting THERM Low as an Input
in Automatic Fan Speed Control Mode
THERM TIMER
The ADT7476 has an internal timer to measure THERM
assertion time. For example, the THERM input can be con-
nected to the PROCHOT output of a Pentium 4 CPU to
measure system performance. The THERM input can also be
connected to the output of a trip point temperature sensor.
The timer is started on the assertion of the ADT7476 THERM
input and stopped when THERM is deasserted. The timer
counts THERM times cumulatively; that is, the timer resumes
counting on the next THERM assertion. The THERM timer
continues to accumulate THERM assertion times until the
timer is read (it is cleared on read) or until it reaches full scale.
If the counter reaches full scale, it stops at that reading until
cleared.
The 8-bit THERM timer register (0x79) is designed so that Bit 0
is set to 1 on the first THERM assertion. Once the cumulative
THERM assertion time has exceeded 45.52 ms, Bit 1 of the
THERM timer is set and Bit 0 now becomes the LSB of the
timer with a resolution of 22.76 ms (see Figure 32).
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