mc68hc05bs8 Freescale Semiconductor, Inc, mc68hc05bs8 Datasheet - Page 54

• Manufacturer Part Number: mc68hc05bs8
• Description: Mc68hc05 Family Of Low-cost Single-chip Microcontrollers.
• Manufacturer: Freescale Semiconductor, Inc

6
6.2
The Core Timer is a 15-stage multi-functional ripple counter which provides miscellaneous
function to the MC68HC05BS8 MCU. It includes a timer overflow function, real-time interrupt, and
COP watchdog.
As seen in Figure 6-6, the Timer is driven by the internal bus clock divided by four with a fixed
prescaler. This signal drives an 8-bit ripple counter. The value of this 8-bit ripple counter can be
read by the CPU at any time by accessing the Ctimer Counter register (CTCR) at address $09. A
timer overflow function is implemented on the last stage of this counter, giving a possible interrupt
at the rate of E/1024. Four additional stages produces a resulting clock of E/16384, driving the
Real Time Interrupt circuit. The RTI circuit consists of three divider stages with a 1 of 4 selector.
The output of the RTI circuit is further divided by eight to drive the optional COP Watchdog Timer
circuit. The RTI rate selector bits, and the RTI and CTOF enable bits and flags are located in the
CTimer Control and Status register (CTCSR) at location $08.
6.2.1
The Core Timer Counter register is a read-only register which contains the current value of the
8-bit ripple counter. This counter is clocked at f
a software capture. Extended time periods can be attained using the TOF function to increment a
temporary RAM storage location thereby simulating a 16-bit (or more) counter.
During the Power-on reset (POR) cycle, all CTimer counters are first cleared, the counters then
count 4064 cycles before it is cleared again. After this 4064 cycles, the POR circuit releases the
device from reset. At this point, if RESET is not asserted, the timer will start counting up from zero
and normal device operation will begin. If RESET is asserted anytime during operation (other than
POR), the counter chain will be cleared.
6.2.2
CTOF - CTimer Overflow
6-10
1 (set)
0 (clear) –
CTCSR
CTCR
CORE TIMER
CTimer Counter Register
CTimer Control and Status Register
–
8-bit ripple timer overflow has occurred.
No 8-bit ripple timer overflow has occurred.
Address bit 7
Address bit 7
$0009
$0008
CTOF
CT7
RTIF CTOFE RTIE
bit 6
bit 6
CT6
TIMERS
OP
/4 and can be used for various functions including
bit 5
bit 5
CT5
bit 4
bit 4
CT4
bit 3
bit 3
CT3
0
bit 2
bit 2
CT2
0
bit 1
bit 1
CT1
RT1
MC68HC05BS8
bit 0
bit 0
CT0
RT0
0000 0000
0000 0011
on reset
on reset
State
State
TPG