Applications Information
10-10
MC68307 USER’S MANUAL
MOTOROLA
Note that since the external clock input to the MC68307 is stopped the device cannot wake-
up from low-power stop mode directly.
Because the MC68307 is fully static, the clock can be stopped at any time and in any phase.
Lowest power will be obtained if the clock is stopped after the device has been put into low-
power sleep mode. If the clock is stopped when the CPU is executing a bus cycle or pro-
cessing an instruction the current will not be minimal. Low-power sleep mode uses arbitra-
tion to ensure that the CPU is idle before stopping the internal clock.
When the clock is restarted, the MC68307 will continue processing from where it was previ-
ously with no loss of status. If the device is in low-power sleep mode an interrupt will be
required to wake-up the CPU.
Instead of stopping the clock externally the frequency could be reduced to a very low value
in order to minimize power. The whole MC68307 could operate at this reduced frequency,
allowing timers for example to wake-up the CPU after a long delay and subsequently set a
general purpose port line to reselect the maximal frequency external clock.
Whenever the external clock is being stopped, started or multiplexed, care must be taken to
ensure that no pulses narrower than the specified minimum clock high or low periods are
generated (minimum pulse width = 27 ns). If an external crystal is stopped then started again
there may be considerable wake-up latency due to the crystal start-up time. This depends
on the external component selection and is not a function of the MC68307.
10.3 USING M-BUS SOFTWARE TO COMMUNICATE BETWEEN
PROCESSOR SYSTEMS
M-bus is an I2C-compatible bus interface used in the MC68300 family. It is a serial interface
comprising two open drain bidirectional signals, namely serial clock (SCL) and serial data
(SDA). Multiple devices can be connected directly to these open drain lines, and indeed this
is good reason for the widespread adoption of the bus as an efficient IC communication
method in end-systems.
A typical scenario would consist of a processor with an M-bus master controlling the data
flow between several slaves, such as LCDs, real-time clocks, keypads, A/D converters and
memories. Moreover, a built-in bus collision mechanism supports multiple M-bus masters as
well as multiple slaves. The M-bus module of the MC68300 is flexible enough to operate as
either an M-bus master or a slave.
This section demonstrates control software for M-bus communication between two identical
MC68307 systems, one configured with an M-bus master and the other an M-bus slave.
Only a short piece of initialization code must be changed to make the MC68307 code appli-
cable to other MC68300 devices with M-bus.