Start Signal; Slave Address Transmission - Motorola MC68HC05T16 Technical Data Manual

MSB
SCL 1 1 0
SDA

START signal

MSB
SCL
1 1 0
SDA
START signal
6.2.1 START Signal
When the bus is free, i.e., no master device is occupying the bus (both SCL and SDA lines are at
logic high), a master may initiate communication by sending a START signal. As shown in
Figure 6-2, a START signal is defined as a high to low transition of SDA while SCL is high. This
signal denotes the beginning of a new data transfer (each data transfer may contain several bytes
of data) and wakes up all slaves.
6.2.2

Slave Address Transmission

The first byte of data transfer immediately following the START signal is the slave address
transmitted by the master. This is a seven bits long calling address followed by a R/W bit. The R/W
bit dictates the slave of the desired direction of data transfer.
Only the slave with matched address will respond by sending back an acknowledge bit by pulling
the SDA low at the 9th clock; see Figure 6-2.
MC68HC05T16
LSB
0 0 0 1 1
Acknowledge bit
LSB
0 0 0 1 1
Acknowledge bit
repeated START signal
Figure 6-2 M-Bus Transmission Signal Diagram
M-BUS SERIAL INTERFACE
MSB
1 1 0 0 0 0
MSB
1 1 0 0 0 0
LSB
1 1
No acknowledge
STOP signal
LSB
1 1
No acknowledge
6
STOP signal
TPG
MOTOROLA
6-3