HT46R46/C46/R47/C47/R48A/C48A/R49
Rev. 1.41
24
December 30, 2008
Programming Considerations
When configured to run in the timer mode, the internal
system clock is used as the timer clock source and is
therefore synchronized with the overall operation of the
microcontroller. In this mode when the appropriate timer
register is full, the microcontroller will generate an internal
interrupt signal directing the program flow to the respec-
tive internal interrupt vector. For the pulse width mea-
surement mode, the internal system clock is also used as
the timer clock source but the timer will only run when the
correct logic condition appears on the external timer input
pin. As this is an external event and not synchronized
with the internal timer clock, the microcontroller will only
see this external event when the next timer clock pulse
arrives. As a result, there may be small differences in
measured values requiring programmers to take this into
account during programming. The same applies if the
timer is configured to be in the event counting mode,
which again is an external event and not synchronized
with the internal system or timer clock.
When the Timer/Event Counter is read, or if data is writ-
ten to the preload register, the clock is inhibited to avoid
errors, however as this may result in a counting error, this
should be taken into account by the programmer. Care
must be taken to ensure that the timers are properly in-
itialised before using them for the first time. The associ-
ated timer enable bits in the interrupt control register must
be properly set otherwise the internal interrupt associated
with the timer will remain inactive. The edge select, timer
mode and clock source control bits in timer control regis-
ter must also be correctly set to ensure the timer is prop-
erly configured for the required application. It is also
important to ensure that an initial value is first loaded into
the timer registers before the timer is switched on; this is
because after power-on the initial values of the timer reg-
isters are unknown. After the timer has been initialised
the timer can be turned on and off by controlling the en-
able bit in the timer control register.
Note that setting the
timer enable bit high to turn the timer on, should only be
executed after the timer mode bits have been properly
setup. Setting the timer enable bit high together with a
mode bit modification, may lead to improper timer oper-
ation if executed as a single timer control register byte
write instruction
.
When the Timer/Event counter overflows, its corre-
sponding interrupt request flag in the interrupt control
register will be set. If the timer interrupt is enabled this
will in turn generate an interrupt signal. However irre-
spective of whether the interrupts are enabled or not, a
Timer/Event counter overflow will also generate a
wake-up signal if the device is in a Power-down condi-
tion. This situation may occur if the Timer/Event Counter
is in the Event Counting Mode and if the external signal
continues to change state. In such a case, the
Timer/Event Counter will continue to count these exter-
nal events and if an overflow occurs the device will be
woken up from its Power-down condition. To prevent
such a wake-up from occurring, the timer interrupt re-
quest flag should first be set high before issuing the
HALT instruction to enter the Power Down Mode.
Timer Program Example
This program example shows how the Timer/Event
Counter registers are setup, along with how the inter-
rupts are enabled and managed. Note how the
Timer/Event Counter is turned on, by setting bit 4 of the
Timer Control Register. The Timer/Event Counter can
be turned off in a similar way by clearing the same bit.
This example program sets the Timer/Event Counter to
be in the timer mode, which uses the internal system
clock as the clock source.
org 04h
; external interrupt vector
reti
org 08h
; Timer/Event Counter interrupt vector
jmp tmrint
; jump here when Timer overflows
:
org 20h
; main program
;internal Timer/Event Counter interrupt routine
tmrint:
:
; Timer/Event Counter main program placed here
:
reti
:
:
begin:
;setup Timer registers
mov a,09bh
; setup Timer preload value
mov tmr,a;
mov a,081h
; setup Timer control register
mov tmrc,a
; timer mode and prescaler set to /2
; setup interrupt register
mov a,005h
; enable master interrupt and timer interrupt
mov intc,a
set tmrc.4
; start Timer - note mode bits must be previously setup
