MC74 Datasheet, PDF (4/12 Pages) ON Semiconductor – Serial Digital Temperature Sensor

MC74
DETAILED OPERATING DESCRIPTION
The MC74 acquires and converts temperature
information from its integrated solid state sensor with a
basic accuracy of ±1°C . It stores the data in an internal
register which is read through the serial port. The system
interface is a slave SMBus. The temperature data can be
read at any time through the SMBus port. Eight SMBus
addresses are programmable for the MC74, which allows for
a multi–sensor configuration. Also, there is low–power
Standby mode where temperature acquisition is suspended.
Standby Mode
The MC74 allows the host to put it into a low power (I_DD
= 5µA, typical) Standby mode. In this mode, the A/D
converter is halted and the temperature data registers are
frozen. The SMBus port operates normally. Standby mode
is enabled by setting the SHDN bit in the CONFIG register.
The table below summarizes this operation.
Standby Mode Operation
SHDN Bit
Operating Mode
0
Normal
1
Standby
SMBus Slave Address
The MC74 is internally programmed to have a default
SMBus address value of 1001 101b. Seven other addresses
are available by custom order (contact factory).
SERIAL PORT OPERATION
The Serial Clock input (SCL) and bi–directional data port
(SDA) form a 2–wire bi–directional serial port for
programming and interrogating the MC74. The following
conventions are used in this bus architecture:
MC74 Serial Bus Conventions
Term Explanation
Transmitter The device sending data to the bus.
Receiver The device receiving data from the bus.
Master
The device which controls the bus: initiating
transfers (START), generating the clock, and
terminating transfers (STOP).
Slave The device addressed by the master.
Start
A unique condition signaling the beginning
of a transfer indicated by SDA falling (High
— Low) while SCL is high.
Stop
A unique condition signaling the end of a
transfer indicated by SDA rising (Low —
High) while SCL is high.
ACK
A receiver acknowledges the receipt of
each byte with this unique condition. The
receiver drives SDA low during SCL high
of the ACK clock–pulse. The Master pro-
vides the clock pulse for the ACK cycle.
Busy
Communication is not possible because
the bus is in use.
NOT Busy When the bus is idle, both SDA and SCL
will remain high.
Data Valid
The state of SDA must remain stable dur-
ing the High period of SCL in order for a
data bit to be considered valid. SDA only
changes state while SCL is low during nor-
mal data transfers (see Start and Stop
conditions).
All transfers take place under control of a host, usually
a CPU or microcontroller, acting as the Master which
provides the clock signal for all transfers. The MC74
always operates as a Slave. The serial protocol is illustrated
in Figure 1. All data transfers have two phases; all bytes are
transferred MSB first. Accesses are initiated by a start
condition (START), followed by a device address byte and
one or more data bytes. The device address byte includes a
Read/Write selection bit. Each access must be terminated by
a Stop Condition (STOP). A convention called
Acknowledge (ACK) confirms receipt of each byte. Note
that SDA can change only during periods when SCL is LOW
(SDA changes while SCL is HIGH are reserved for Start and
Stop Conditions).
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