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TC74-4 3/26/99
Tiny Serial Digital Thermal Sensor
TC74
2001 Microchip Technology Inc.
DS21462A
Start Condition (START)
The TC74 continuously monitors the SDA and SCL lines
for a start condition (a HIGH to LOW transition of SDA while
SCL is HIGH), and will not respond until this condition is met.
Address Byte
Immediately following the Start Condition, the host must
transmit the address byte to the TC74. The states of A1 and
A0 determine the 7-bit SMBus address for the TC74. The
7-bit address transmitted in the serial bit stream must match
for the TC74 to respond with an Acknowledge (indicating the
TC74 is on the bus and ready to accept data). The eighth bit
in the Address Byte is a Read-Write Bit. This bit is a 1 for a
read operation or 0 for a write operation. During the first
phase of any transfer this bit will be set = 0 to indicate that
the command byte is being written.
Acknowledge (ACK)
Acknowledge (ACK) provides a positive handshake
between the host and the TC74. The host releases SDA
after transmitting eight bits, then generates a ninth clock
cycle to allow the TC74 to pull the SDA line LOW to
acknowledge that it successfully received the previous eight
bits of data or address.
Data Byte
After a successful ACK of the address byte, the host
must transmit the data byte to be written or clock out the data
to be read. (See the appropriate timing diagrams. ) ACK will
be generated after a successful write of a data byte into the
TC74.
Stop Condition (STOP)
Communications must be terminated by a stop condi-
tion (a LOW to HIGH transition of SDA while SCL is HIGH).
The Stop Condition must be communicated by the transmit-
ter to the TC74. NOTE: Refer to Timing Diagrams for serial
bus timing.
REGISTER SET AND PROGRAMMER’S
MODEL
TC74 Command Set
(SMBus READ_BYTE and WRITE_BYTE)
COMMAND BYTE DESCRIPTION
COMMAND
CODE
FUNCTION
RTR
00h
Read Temperature (TEMP)
RWCR
01h
Read/Write Configuration (CONFIG)
Configuration Register (CONFIG),
8-BITS, READ/WRITE
CONFIGURATION REGISTER (CONFIG)
D[7]
D[6]
D[5]
D[4]
D[3]
D[2]
D[1]
D[0]
SHDN
DATA RDY
RESERVED
BIT
POR
FUNCTION
TYPE
OPERATION
D[7]
0
STANDBY
ReadWrite
1 = standby,
switch
0 = normal
D[6]
0
Data Ready*
Read Only
1 = ready,
0 = not ready
D[5]-D[0]
0
Reserved -
N/A
Always returns
zero when read.
Temperature Register (TEMP),
8-Bits, READ-ONLY
The binary value (2’s complement format) in this regis-
ter represents temperature of the on-board sensor following
a conversion cycle. The registers are automatically updated
in an alternating manner.
Temperature Register (TEMP)
D[7]
D[6]
D[5]
D[4]
D[3]
D[2]
D[1]
D[0]
MSB
X
LSB
*DATA_RDY bit reset at power-up and SHDN enable (see below).
Figure 2. DATA_RDY, SHDN Operation Logic Diagram
VDD
DATA_RDY
SHDN
tconv