ADT7475 データシートの表示（PDF） - ON Semiconductor
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ADT7475 Datasheet PDF : 58 Pages
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ADT7475
connected to a common SMBALERT line connected to the
master. If a device’s SMBALERT line goes low, the
following events occur:
1. SMBALERT is pulled low.
2. The master initiates a read operation and sends the
alert response address (ARA = 0001 100). This
general call address must not be used as a specific
device address.
3. The device whose SMBALERT output is low
responds to the alert response address, and the
master reads its device address. The address of the
device is now known and can be interrogated in
the usual way.
4. If more than one device’s SMBALERT output is
low, the one with the lowest device address has
priority in accordance with normal SMBus
arbitration.
5. Once the ADT7475 has responded to the alert
response address, the master must read the status
registers, and the SMBALERT is cleared only if
the error condition has gone away.
SMBus Timeout
The ADT7475 includes an SMBus timeout feature. If
there is no SMBus activity for 35 ms, the ADT7475 assumes
that the bus is locked and releases the bus. This prevents the
device from locking or holding the SMBus expecting data.
Some SMBus controllers cannot handle the SMBus timeout
feature, so it can be disabled.
Configuration Register 1 (0x40)
Bit 6 TODIS = 0; SMBus timeout enabled (default).
Bit 6 TODIS = 1; SMBus timeout disabled.
Virus Protection
To prevent rogue programs or viruses from accessing
critical ADT7475 register settings, the lock bit can be set.
Setting Bit 1 of Configuration Register 1 (0x40) sets the lock
bit and locks critical registers. In this mode, certain registers
can no longer be written to until the ADT7475 is powered
down and powered up again. For more information on which
registers are locked, see the Register Tables section.
Voltage Measurement Input
The ADT7475 has one external voltage measurement
channel. It can also measure its own supply voltage, VCC.
Pin 14 can measure VCCP. The VCC supply voltage
measurement is carried out through the VCC pin (Pin 3). The
VCCP input can be used to monitor a chipset supply voltage
in computer systems.
Analog−to−Digital Converter
All analog inputs are multiplexed into the on−chip,
successive approximation, analog−to−digital converter.
This has a resolution of 10 bits. The basic input range is 0 V
to 2.25 V, but the input has built−in attenuators to allow
measurement of VCCP without any external components. To
allow for the tolerance of the supply voltage, the ADC
produces an output of 3/4 full scale (decimal 768 or 300 hex)
for the nominal input voltage and so has adequate headroom
to deal with overvoltages.
Input Circuitry
The internal structure for the VCCP analog input is shown
in Figure 19. The input circuit consists of an input protection
diode, an attenuator, and a capacitor to form a first−order,
low−pass filter that gives the input immunity to high
frequency noise.
VCCP
17.5kW
52.5kW
35pF
Figure 19. Structure of Analog Inputs
Voltage Measurement Registers
Register 0x21, VCCP Reading = 0x00 default
Register 0x22, VCC Reading = 0x00 default
VCCP Limit Registers
Associated with the VCCP measurement channel is a high
and low limit register. Exceeding the programmed high or
low limit causes the appropriate status bit to be set.
Exceeding either limit can also generate SMBALERT
interrupts.
Register 0x46, VCCP Low Limit = 0x00 default
Register 0x47, VCCP High Limit = 0xFF default
Table 2 shows the input ranges of the analog inputs and
output codes of the 10−bit ADC.
When the ADC is running, it samples and converts a
voltage input in 711 ms and averages 16 conversions to
reduce noise; a measurement takes nominally 11.38 ms.
Extended Resolution Registers
Voltage measurements can be made with higher accuracy
using the extended resolution registers (0x76 and 0x77).
Whenever the extended resolution registers are read, the
corresponding data in the voltage measurement registers is
locked until their data is read. That is, if extended resolution
is required, then the extended resolution register must be
read first, immediately followed by the appropriate voltage
measurement register.
Additional ADC Functions for Voltage Measurements
A number of other functions are available on the
ADT7475 to offer the system designer increased flexibility.
Turn−Off Averaging
For each voltage measurement read from a value register,
16 readings have been made internally, and the
results averaged, before being placed into the value register.
For instances where faster conversions are needed, setting
Bit 4 of Configuration Register 2 (0x73) turns averaging off.
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