MOTOROLA
9-30
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
M68HC11
REFERENCE MANUAL
method of wake up allows messages to include gaps of idle time, unlike the earlier idle-
line method, but there is an efficiency loss due to the extra bit time per character (ad-
dress bit) required in all characters.
9.5 Baud-Rate Generator
This discussion, which provides a more detailed description of the baud-rate genera-
tor, is useful to those users who have an odd crystal frequency or some other unusual
requirement. The usual approach to selecting a baud rate is to start from a particular
crystal frequency and select one of the 32 available division factors to arrive at a de-
sired baud rate. If a system uses equipment from other manufacturers (such as a CRT
terminal or host computer), a standard baud rate, such as 9600 baud, is used. Many
applications include SCI networks that do not have to communicate with ordinary
RS232-type sources. In these cases, any frequency can be used as long as all of the
devices in the network use the same baud rate — for example, in the automotive in-
dustry, 32.768K baud has commonly been used. These systems include separate
controllers for various parts of the car and test equipment interfaces. All of the custom
equipment is controlled by the automobile manufacturer. In this environment, high
throughput on the SCI and maximum bus frequency are more important than a cus-
tomary baud rate.
9.5.1 Timing Chain Block Diagram
Figure 9-12
is a block diagram of the SCI baud-rate timing chain. The crystal frequen-
cy is divided by four to get the bus rate PH2 clock. PH2 is the same frequency as E
(but leads E by 90
°
). The prescale select bits, SCP[1:0], select an initial division factor
of 1, 3, 4, or 13 that drives a series of divide-by-two stages. The rate select bits
(SCR[2:0]) determine where the RT clock will be tapped off of this divider chain. The
RT clock is 16 times the baud-rate frequency. This 16 x baud-rate clock is finally divid-
ed by 16 to get the transmitter baud-rate clock. The actual baud-rate generator func-
tions like this block diagram although the implementation is slightly different to
overcome potential circuit-delay problems. (It is important to the overall SCI circuit de-
sign to maintain minimum delays from PH2 to baud rate clock edges.)
9.5.2 Baud Rates vs. Crystal Frequency
Table 9-3
is a complete listing of all possible baud rates that can be achieved for five
crystal frequencies. The entries most likely to be of interest are in bold.
9.6 SCI Timing Details
The timing information presented in the following paragraphs is much more detailed
than most users need but will help normal users better understand the SCI system.
For rare applications needing this much detail, this discussion is an authoritative ref-
erence source although it is not intended to replace guaranteed data-sheet timing in-
formation.