
MOTOROLA
10-6
MAIN TIMER AND REAL-TIME INTERRUPT
M68HC11
REFERENCE MANUAL
each other.
There is a relatively complex block of logic that divides the 4 x oscillator clock down to
the internal phase 2 (PH2) clocks and the external E clock. The address strobe (AS)
signal for demultiplexing the low-order address from data is also developed in this first
oscillator divider block. Almost everything that happens inside the MCU is referenced
to the internal PH2 clocks rather than the E clock, which lags 90
°
behind the internal
PH2 clocks. Users who are familiar with the older MC6800 and MC6801 Families
should note that this phase shift between E and PH2 is different from what they are
used to. From an external point of view, they can still think of bus cycles as starting
and ending on falling edges of E, but they will notice a big improvement in address and
data hold times relative to this edge.
Figure 10-2
shows idealized timing relationships for the clocks and AS that are devel-
oped in the initial oscillator divider block. Since this section is devoted to the main timer
system, these clocks will not be discussed in any great detail;
Figure 10-2
is presented
for reference only. For more information concerning these signals, refer to
SECTION
2 PINS AND CONNECTIONS
and
Figure 7-12
.
The logic associated with the STOP power-saving mode also contributes to the com-
plexity of the initial oscillator divider block. When the STOP mode is exited, the internal
clocks resume before the external E clock starts.
The pulse accumulator can be clocked by an external source (event counting mode)
or an internal source (time accumulation mode). The internal clocking source is an E
divided by 64 rate clock, which is tapped off of the main timer clocking chain. The pulse
accumulator is described in more detail in
SECTION 11 PULSE ACCUMULATOR
, but
the E divided by 64 tap is discussed in this section since it is tapped off the main timer.
The pulse accumulator tap is also used to inhibit write permission to the time-protected
control registers and bits. Certain registers and bits, such as the timer prescaler control
bits in the TMSK2 register, can only be written within the first 64 E-clock cycles after
reset.
Figure 10-2 Timing Summary for Oscillator Divider Signals
ONE BUS CYCLE
EXTAL
PH2
E
AS
ADDRESS/DATA
DATA
ADDR