Technical Data
MC68HC11E Family
—
Rev. 4
180
Timing System
MOTOROLA
Timing System
9.3 Timer Structure
Figure 9-2
shows the capture/compare system block diagram. The
port A pin control block includes logic for timer functions and for
general-purpose I/O. For pins PA3, PA2, PA1, and PA0, this block
contains both the edge-detection logic and the control logic that enables
the selection of which edge triggers an input capture. The digital level on
PA[3:0] can be read at any time (read PORTA register), even if the pin
is being used for the input capture function. Pins PA[6:3] are used for
either general-purpose I/O, or as output compare pins. When one of
these pins is being used for an output compare function, it cannot be
written directly as if it were a general-purpose output. Each of the output
compare functions (OC[5:2]) is related to one of the port A output pins.
Output compare one (OC1) has extra control logic, allowing it optional
control of any combination of the PA[7:3] pins. The PA7 pin can be used
as a general-purpose I/O pin, as an input to the pulse accumulator, or as
an OC1 output pin.
Table 9-1. Timer Summary
XTAL Frequencies
Control Bits
PR1, PR0
4.0 MHz
8.0 MHz
12.0 MHz
Other Rates
1.0 MHz
2.0 MHz
3.0 MHz
(E)
1000 ns
500 ns
333 ns
(1/E)
Main Timer Count Rates
0 0
1 count
—
overflow
—
1000 ns
65.536 ms
500 ns
32.768 ms
333 ns
21.845 ms
(E/1)
(E/2
16
)
0 1
1 count
—
overflow
—
4.0
μ
s
262.14 ms
2.0
μ
s
131.07 ms
1.333
μ
s
87.381 ms
(E/4)
(E/2
18
)
1 0
1 count
—
overflow
—
8.0
μ
s
524.29 ms
4.0
μ
s
262.14 ms
2.667
μ
s
174.76 ms
(E/8)
(E/2
19
)
1 1
1 count
—
overflow
—
16.0
μ
s
1.049 s
8.0
μ
s
524.29 ms
5.333
μ
s
349.52 ms
(E/16)
(E/2
20
)