TMP421 datasheet(18/30 Pages) TI

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READ/WRITE OPERATIONS

TMP421

TMP422

TMP423

SBOS398B – JULY 2007 – REVISED MARCH 2008

The TMP422 checks the polarity of the external

transistor at power-on, or after software reset, by

Accessing a particular register on the TMP421/22/23

forcing current to pin 1 while connecting pin 2 to

is accomplished by writing the appropriate value to

approximately 0.6V. If the voltage on pin 1 does not

the Pointer Register. The value for the Pointer

pull up to near the V+ of the TMP422, pin 1 functions

as DXP for channel 1, and the second LSB of the

address byte with the R/W bit low. Every write

slave address is '0'. If the voltage on pin 1 does pull

operation to the TMP421/22/23 requires a value for

up to near V+, the TMP422 forces current to pin 2

the Pointer Register (see Figure 17).

while connecting pin 1 to 0.6V. If the voltage on pin 2

does not pull up to near V+, the TMP422 uses pin 2

When reading from the TMP421/22/23, the last value

for DXP of channel 1, and sets the second LSB of the

stored in the Pointer Register by a write operation is

slave address to '1'. If both pins are shorted to GND

used to determine which register is read by a read

or if both pins are open, the TMP422 uses pin 1 as

operation. To change which register is read for a read

DXP and sets the address bit to '0'. This process is

operation, a new value must be written to the Pointer

then repeated for channel 2 (pins 3 and 4).

a slave address byte with the R/W bit low, followed

If the TMP422 is to be used with transistors that are

by the Pointer Register byte; no additional data are

located on another IC (such as a CPU, DSP, or

required. The master can then generate a START

graphics processor), it is recommended to use pin 1

condition and send the slave address byte with the

or pin 3 as DXP to ensure correct address detection.

R/W bit high to initiate the read command. See

If the other IC has a lower supply voltage or is not

Figure 19 for details of this sequence. If repeated

powered when the TMP422 tries to detect the slave

reads from the same register are desired, it is not

address, a protection diode may turn on during the

necessary to continually send the Pointer Register

detection process and the TMP422 may incorrectly

bytes, because the TMP421/22/23 retain the Pointer

choose

the

DXP

and

corresponding

slave

address. Using pin 1 and/or pin 3 for transistors that

operation. Note that register bytes are sent MSB first,

are

other

ICs

ensures

correct

operation

followed by the LSB.

independent of supply sequencing or levels.

Read operations should be terminated by issuing a

The TMP423 has a factory-preset slave address. The

Not-Acknowledge command at the end of the last

TMP423A slave address is 1001100b, and the

byte to be read. For a single-byte operation, the

TMP423B

slave

address

1001101b.

The

master should leave the SDA line high during the

configuration of the DXP and DXN channels are

Acknowledge time of the first byte that is read from

independent of the address. Unused DXP channels

the slave. For a two-byte read operation, the master

can be left open or tied to GND.

must pull SDA low during the Acknowledge time of

the first byte read, and should leave SDA high during

the Acknowledge time of the second byte read from

the slave.

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