Clock Synchronization; Handshaking; M-Bus Registers - Motorola MC68HC05T16 Technical Data Manual

6.2.7

Clock Synchronization

Since wire-AND logic is performed on the SCL line, a high to low transition on SCL line will affect
the devices connected to the bus. The devices start counting their low period and once a device's
clock has gone low, it will hold the SCL line low until the clock high state is reached. However, the
change of low to high in this device clock may not change the state of the SCL line, if another
device clock is still in its low period. Therefore synchronized clock SCL will be held low by the
device which releases SCL to a logic high in the last place. Devices with shorter low periods enter
a high wait state during this time (See Figure 6-3). When all devices concerned have counted off
their low period, the synchronized clock SCL line will be released and go high. All of them will start
counting their high periods. The first device to complete its high period will again pull the SCL line
low.
SCL1
SCL2
6.2.8

Handshaking

The clock synchronization mechanism can be used as a handshake in data transfer. Slave device
may hold the SCL low after completion of one byte transfer (9 bits). In such case, it will halt the
bus clock and force the master clock in a wait state until the slave releases the SCL line.
6.3

M-Bus Registers

There are five registers used in the M-Bus interface, these are discussed in the following
paragraphs.
MC68HC05T16
WAIT
SCL
Internal counter reset
Figure 6-3 Clock Synchronization
M-BUS SERIAL INTERFACE
Start counting high period
6
TPG
MOTOROLA
6-5