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Serial Bus Interface
Control of the ADT7476A is carried out using the serial
system management bus (SMBus). The ADT7476A is
connected to this bus as a slave device, under the control of
a master controller. The ADT7476A has a 7-bit serial bus
address. When the device is powered up with Pin 13
(PWM3/ADDREN) high, the ADT7476A has a default
SMBus address of 0101110 or 0x2E. The read/write bit must
be added to get the 8-bit address. If more than one
ADT7476A is to be used in a system, each ADT7476A is
placed in ADDR SELECT mode by strapping Pin 13 low on
powerup. The logic state of Pin 14 then determines the
device's SMBus address. The logic of these pins is sampled
on powerup.
The device address is sampled on powerup and latched on
the first valid SMBus transaction, more precisely on the
low-to-high transition at the beginning of the eighth SCL
pulse, when the serial bus address byte matches the selected
slave address. The selected slave address is chosen using the
ADDREN pin/ADDR SELECT pin. Any attempted
changes in the address have no effect after this.
Table 5. HARDWIRING THE ADT7476A SMBUS
DEVICE ADDRESS
Pin 13 State
0
0
1
ADDR SELECT
Figure 14. Default SMBus Address = 0x2E
ADDR SELECT
Figure 15. SMBus Address = 0x2C (Pin 14 = 0)
ADDR SELECT
Figure 16. SMBus Address = 0x2D (Pin 14 = 1)
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Pin 14 State
Address
Low (10 kW to GND)
0101100 (0x2C)
High (10 kW Pullup)
0101101 (0x2D)
Don't Care
0101110 (0x2E)
V
CC
ADT7476A
14
10 kW
13
PWM3/
ADDREN
ADDRESS = 0x2E
ADT7476A
10 kW
14
13
PWM3/
ADDREN
ADDRESS = 0x2C
V
CC
ADT7476A
10 kW
14
13
PWM3/
ADDR_EN
ADDRESS = 0x2D
ADT7476A
ADT7476A
ADDR SELECT
PWM3/
ADDREN
CARE SHOULD BE TAKEN TO ENSURE THAT PIN 13
(PWM3/ADDREN) IS EITHER TIED HIGH OR LOW. LEAVING
PIN 13 FLOATING COULD CAUSE THE ADT7476A TO POWER UP
WITH AN UNEXPECTED ADDRESS.
NOTE THAT IF THE ADT7476A IS PLACED INTO ADDR SELECT
MODE, PINS 13 AND 14 CANNOT BE USED AS THE ALTERNATE
FUNCTIONS (PWM3, TACH4/THERM) UNLESS THE CORRECT
CIRCUIT IS MUXED IN AT THE CORRECT TIME OR DESIGNED
TO HANDLE THESE DUAL FUNCTIONS.
Figure 17. Unpredictable SMBus Address if Pin 13
The ability to make hardwired changes to the SMBus
slave address allows the user to avoid conflicts with other
devices sharing the same serial bus, for example, if more
than one ADT7476A is used in a system.
The serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a
start condition, which is defined as a high-to-low
transition on the serial data line SDA while the
serial clock line SCL remains high. This indicates
that an address/data stream follows. 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 a
R/W bit, which determine the direction of the data
transfer, that is, whether data is 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.
2. Data is sent over the serial bus in sequences of
nine clock pulses, eight bits of data followed by an
acknowledge bit from the slave device. Transitions
on the data line must occur during the low period
of the clock signal and remain stable during the
high period. A low-to-high transition, when the
clock is high, can be interpreted as a stop signal.
The number of data bytes transmitted over the
serial bus in a single read or write operation is
limited only by what the master and slave devices
can handle.
3. When all data bytes have been read or written,
stop conditions are established. In write mode, the
master pulls the data line high during the 10
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9
V
CC
10 kW
14
13
DO NOT LEAVE ADDREN
NC
UNCONNECTED! CAN
CAUSE UNPREDICTABLE
ADDRESSES.
is Unconnected
th
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