ADT7476ARQZ-R7 datasheet(10/67 Pages) ONSEMI

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ADT7476

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Serial Bus Interface

Control of the ADT7476 is carried out using the serial

system management bus (SMBus). The ADT7476 is

connected to this bus as a slave device, under the control of

a master controller. The ADT7476 has a 7−bit serial bus

address. When the device is powered up with Pin 13

(PWM3/ADDREN) high, the ADT7476 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 ADT7476

is to be used in a system, each ADT7476 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 1. Hardwiring the ADT7476 SMBus Device

Address

Pin 13 State

Pin 14 State

Address

Low (10 kW to GND)

0101100 (0x2C)

High (10 kW pullup)

0101101 (0x2D)

Don’t care

0101110 (0x2E)

Figure 14. Default SMBus Address = 0x2E

ADT7476

ADDRESS = 0x2E

VCC

ADDR SELECT

PWM3/ADDREN

10kW

Figure 15. SMBus Address = 0x2C (Pin 14 = 0)

ADT7476

ADDRESS = 0x2C

10k

ADDR SELECT

PWM3/ADDREN

Figure 16. SMBus Address = 0x2D (Pin 14 = 1)

ADT7476

ADDRESS = 0x2D

VCC

10k

ADDR SELECT

PWM3/ADDREN

Figure 17. Unpredictable SMBus Address if Pin 13

is Unconnected

DO NOT LEAVE ADDREN

UNCONNECTED! CAN

CAUSE UNPREDICTABLE

ADDRESSES.

ADT7476

VCC

10k

CARE SHOULD BE TAKEN TO ENSURE THAT PIN 13

(PWM3/ADDREN) IS EITHER TIED HIGH OR LOW. LEAVING PIN 13

FLOATING COULD CAUSE THE ADT7476 TO POWER UP WITH AN

UNEXPECTED ADDRESS.

NOTE THAT IF THE ADT7476 IS PLACED INTO ADDR SELECT

MODE, PINS 13 AND 14 CANNOT BE USED AS THE ALTERNATIVE

FUNCTIONS (PWM3, TACH4/THERM) UNLESS THE CORRECT

CIRCUIT IS MUXED IN AT THE CORRECT TIME OR DESIGNED TO

HANDLE THESE DUAL FUNCTIONS.

ADDR SELECT

PWM3/ADDREN

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 ADT7476 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 10th clock

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