Timing System
Timer Structure
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Timing System
143
The COP watchdog clock input (E
÷
2
15
) is tapped off of the free-running counter
chain. The COP automatically times out unless it is serviced within a specific time
by a program reset sequence. If the COP is allowed to time out, a reset is
generated, which drives the RESET pin low to reset the MCU and the external
system. Refer to
Table 9-1
for crystal-related frequencies and periods.
9.2 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
)
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.