ADT7468
Rev. A | Page 10 of 84
PRODUCT DESCRIPTION
The ADT7468 is a complete thermal monitor and multiple fan
controller for any system requiring thermal monitoring and
cooling. The device communicates with the system via a serial
system management bus. The serial bus controller has 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 for the ADT7468 are performed over
the serial bus. In addition, a pin can be reconfigured as an
SMBALERT output to signal out-of-limit conditions.
COMPARISON BETWEEN ADT7463 AND ADT7468
The ADT7468 is an upgrade to the ADT7463. The ADT7468
and ADT7463 are almost pin and register map compatible. The
ADT7468 and ADT7463 have the following differences:
1. On the ADT7468, the PWM drive signals can be config-
ured as either high frequency or low frequency drives. The
low frequency option is programmable between 10 Hz and
100 Hz. The high frequency option is 22.5 kHz. On the
ADT7463, only the low frequency option is available.
2. Once VCC is powered up, monitoring of temperature and
fan speeds is enabled on the ADT7468 when VCCP is
powered up. If VCCP is never powered up, this is enabled
when the first SMBus transaction with the ADT7468 is
completed. On the ADT7463, the STRT bit in Configur-
ation Register 1 must be set to enable monitoring.
3. The fans are switched off by default upon power-up. On
the ADT7463, the fans run at full speed on power-up.
Fail-safe cooling is provided such that when the measured
temperature exceeds the THERM limit (100°C), the fans
run at full speed.
Fail-safe cooling is also provided 4.6 secs after VCCP is
powered-up (see Figure 48). The fans operate at full speed
if the ADT7468 has not been addressed via the SMBus
within 4.6 secs of when the VCCP is powered up. This
protects the system in the event that the SMBus fails. The
ADT7468 can be programmed at any time, and it behaves
as programmed. If VCCP is never powered-up, fail-safe
cooling is effectively disabled. If VCCP is disabled, writing to
the ADT7468 at any time causes the ADT7468 to operate
normally.
4. Series resistance cancellation (SRC) is provided on the
remote temperature channels on the ADT7468, but not on
the ADT7463. SRC automatically cancels linear offset
introduced by a series resistance between the thermal
diode and the sensor.
5. The ADT7468 has an extended temperature measurement
range. The measurement range goes from–64°C to +191°C.
On the ADT7463, the measurement range is from −127°C
to +127°C. This means that the ADT7468 can measure
higher temperatures. The ADT7468 also includes the
ADT7463 temperature range; the temperature measure-
ment range can be switched by setting Bit 0 of
Configuration Register 5.
6. The ADT7468 maximum fan speed (% duty cycle) in the
automatic fan speed control loop can be programmed. The
maximum fan speed is 100% duty cycle on the ADT7463
and is not programmable.
7. The offset register in the ADT7468 is programmable up
to ±64°C with 0.50°C resolution. The offset register of
the ADT7463 is programmable up to ±32°C with
0.25°C resolution.
8. VCCP is monitored on Pin 23 of the ADT7468 and can be
used to set the threshold for THERM (PROCHOT) (2/3 of
VCCP). The threshold for THERM (PROCHOT) is set at
VIH = 1.7 V and VIL = 0.8 V on the ADT7463.
9. On the ADT7463, Pin 22 can be reconfigured as SMBus
ALERT. This is not available on the ADT7468; instead,
SMBALERT can be enabled on Pin 14.
10. A GPIO can also be made available on Pin 14 on the
ADT7468. This is not available on the ADT7463. Set the
GPIO polarity and direction in Configuration Register 5.
The GPIO status bit is Bit 5 of Status Register 2 (shared
with TACH4 and THERM, because only one can be
enabled at a time).
11. The ADT7463 has three possible SMBus addresses, which
are selectable using the address select and address enable
pins. The ADT7468 has one SMBus address available at
Address 0x2E.
Due to the inclusion of extra functionality, the register map has
changed, including an additional configuration register:
Configuration Register 5 at Address 0x7C.
Configuration Register 5
Bit 0: If Bit 0 is set to 1, in terms of temperature the ADT7468 is
backward-compatible with the ADT7463. Measurements
including the TMIN calibration circuit, and fan control work in
the range of −127°C to +127°C. Also, care should be taken in
reprogramming the temperature limits (TMIN, operating point,
and THERM limits) to their desired twos complement value,
because the power-on default for them is at Offset 64. The
extended temperature range is −64°C to 191°C. The default is 1,
which is in the −64°C to +191°C temperature range.
Bit 1= 0 is the high frequency (22.5 kHz) fan drive signal.
