
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
.