Page 182 - Programming Microcontrollers in C

Timers 167

offered in some microcontrollers is the computer operating prop­
erly (COP) timer. Each of the systems will be examined in some
detail in the following paragraphs.

Multifunction Timer—15-Bit Timer
A timer of this nature can be found on the smallest of the
M68HC05 components, such as the M68HC05J1 or the M68HC05P8,
and it is not found on the M68HC05Bx devices discussed previously.
This timer consists of an 8-bit ripple counter followed by an addi­
tional 7-bit counter. This counter chain is driven by a signal that is at
one-fourth the internal clock frequency of the microcontroller. The
internal clock frequency in turn is half the crystal frequency. This
portion of the counter is completely uncontrolled. The program can,
however, read the value of this counter at any time. A block diagram
of this type of timer is shown in Figure 4-1.
Internal
Processor
Clock
(XTAL ÷2)
Least Significant Eight Bits of 15 Stage Ripple Counter
Fixed
÷2 ÷2 ÷2 ÷2 ÷2 ÷2 ÷2 ÷2 Divide By
4
MSB LSB
$0009
Timer Count Register
TCR

$0008
TOF RTIF TOFE RTIE 0 0 RT1 RT0
TCSR
RTI Rate Select

÷2 ÷2 ÷2 ÷2 ÷2 ÷2 ÷2

Most Significant Seven Bits of 15 Stage Ripple Counter

÷2 ÷2 ÷2 S
Q COP Timeout-Generate
Internal MCU Reset
Service (Clear) R
COP Watchdog

Figure 4-1 15-bit Timer Block Diagram