Internal Memory Configuration and External Expansion - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 728

together and represented as parallel lines with crossovers, as illustrated in Figure 14.27. A cross-
over in the waveform represents the point at which one or multiple signals change values. The
HCS12 literature shows that Freescale uses 0.65 V CC and 0.35 V CC as high input voltage ( V IH )
and low input voltage ( V IL ). All timing indications use these two values as the reference points.

1

0

Figure 14.26 ■ Single-signal waveform

One or multiple signals
change value

1

0

Figure 14.27 ■ Multiple-signal waveform

Sometimes a signal value is unknown because the signal is changing. Hatched areas in
the timing diagram, shown in Figure 14.28, represent single and multiple unknown signals.
Sometimes one or multiple signals are not driven (because their drivers are not enabled) and
hence cannot be received. An undriven signal is said to be floating or in a high-impedance state.
Single and multiple floating signals are represented by a value between the high and low levels,
as shown in Figure 14.29.
In a microcontroller system, a bus signal falls into one of the three categories: address ,
data , or control .

Unknown

(a)

Unknown

(b)

Figure 14.28 ■ Unknown signals: (a) single signal,
(b) multiple signal