TMP141
SBAS347A MARCH 2005 REVISED JULY 2006
www.ti.com
8
DIGITAL INTERFACE
Up to four TMP141 devices can be connected to a single
signaling line, which is called Single-Wire Data (SWD).
This line is similar to the data line (SMBDAT) of the
industry-standard SMBus. The SensorPath bus uses a
power supply of 3.3V; therefore, the signaling levels are
approximately 3.3V and 0V.
Data is encoded in a pulse width encoding scheme. All
signaling is done by pulling the bus low for varying lengths
of time. With the exception of Reset and Attention
Request, the master starts all bits. Thus, when sending
data, the TMP141 does not pull the bus low. Rather, when
the TMP141 needs to send a 1, it first detects that the line
has been pulled low by the master, and then holds the line
low long enough to signal a 1. To send a 0, the TMP141
simply leaves the bus alone; the master has already
placed a 0 on the bus.
The SWD interface used on the TMP141 has five types of
signals that are identified by the length of time that the line
is held low. Conceptually, this device operates as a
five-state signaling system. The more common signals
have shorter durations in order to speed overall
transmission. The five signals and their approximate times
are:
D  Data 0: Transmit a 0 bit (15?/SPAN>s)
D   Data 1: Transmit a 1 bit (42?/SPAN>s)
D   Start: Used by the master at the beginning of a
transaction (95祍)
D   Attention Request: May be used by the TMP141 to
request service (196祍)
D   Reset: Must be asserted by both the master and
slave(s) after power up. It may be asserted at other
times as needed (at least 348祍).
The bus must be inactive for at least 11祍 before any
signal.
Start
All devices on the bus continuously monitor the bus. The
master also monitors the bus to verify that it is inactive
before issuing a Start Bit. During the Start Bit, both the
master and slave(s) monitor the bus to detect an Attention
Request or a Reset. If there is an Attention Request, or a
Reset, the current signal is not treated as a Start Bit. The
master may reissue the Start Bit at a later time, but is not
required to.
Attention Request
Any slave device may signal an Attention Request. Before
sending an Attention Request, the slave monitors the bus
to ensure that it is inactive. If an Attention Request
coincides with a data bit from another device or a Start Bit
from the master, the Attention Request will take
precedence since the Attention Request signal holds the
bus low longer. The master and all communicating slaves
will monitor the bus to detect an Attention Request. The
master monitors the bus in order to allow resending, in
case the Attention Request started simultaneously with a
Start Bit or a data bit. If the master was in the midst of a
transaction when the Attention Request was asserted, the
master can either continue with the transaction by
repeating the bit that was extended by the Attention
Request, initiating the next bit if a bit was not extended, or
it can service the Attention Request by issuing a new start
command for a new transaction.
Power-On Reset
All devices on the SWD bus, including the TMP141, are
required to generate a Reset signal at power up. Reset
aborts any operation in progress, and sets all registers to
the default values. The device that holds SWD low the
longest will define how long the Reset lasts. After a Reset,
a slave device may not send an Attention Request until the
master has sent at least 14 bits of data on the SWD bus.
Device Reset
The TMP141 will also Reset when the Reset bit is written
to the Device Control Register (bit 0). All registers are set
to the default value. The two least significant bits of the
Device Control Register are the only locations that
respond to a broadcast write command from the master.
A software-programmed reset of a device by writing to its
Device Control Register will not cause a Reset signal on
the bus.
BUS RESET
A Bus Reset occurs when the TMP141 detects a Reset
signal on the SWD bus. Upon Bus Reset, the TMP141 will
abandon   any   communication   in   progress.   Internal
registers are not reset to their power-up values; thus, any
error flags such as BER or ORUN will remain unaffected.
If a conversion was in progress, it will continue to
completion.