HT46R51/HT46R52
Timer/Event Counter
flows, the counter is reloaded from the timer/event coun-
ter register and issues an interrupt request, as in the
other two modes, i.e., event and timer modes.
Only one timer/event counter (TMR) are implemented in
the microcontroller. The timer/event counter contains an
8-bit programmable count-up counter and the clock may
come from an external source or an internal clock
source. An internal clock source comes from fSYS. The
external clock input allows the user to count external
events, measure time intervals or pulse widths, or to
generate an accurate time base.
To enable the counting operation, the Timer ON bit
(TON; bit 4 of the TMRC) should be set to ²1². In the
pulse width measurement mode, the TON is automati-
cally cleared after the measurement cycle is completed.
But in the other two modes, the TON can only be reset
by instructions. The overflow of the timer/event counter
is one of the wake-up sources and can also be applied
to a PFD (Programmable Frequency Divider) output at
PA3 by options. No matter what the operation mode is,
writing a ²0² to ETI (bit2 of the INTC) disables the re-
lated interrupt service. When the PFD function is se-
lected, executing ²SET [PA].3² instruction to enable the
PFD output and executing ²CLR [PA].3² instruction to
disable the PFD output.
There are two registers related to the Timer/event coun-
ter; TMR (0DH), TMRC (0EH). Writing TMR will transfer
the specified data to timer/event counter registers.
Reading the TMR will read the contents of the
timer/event counter. The TMRC is a control register,
which defines the operating mode, counting enable or
disable and an active edge.
The TM0 and TM1 bits define the operation mode. The
event count mode is used to count external events,
which means that the clock source is from an external
(TMR) pin. The timer mode functions as a normal timer
with the clock source coming from the internal selected
clock source. Finally, the pulse width measurement
mode can be used to count the high or low level duration
of the external signal (TMR), and the counting is based
on the internal selected clock source.
In the case of timer/event counter OFF condition, writing
data to the timer/event counter preload register also re-
loads that data to the timer/event counter. But if the
timer/event counter is turn on, data written to the
timer/event counter is kept only in the timer/event coun-
ter preload register. The timer/event counter still contin-
ues its operation until an overflow occurs.
In the event count or timer mode, the timer/event coun-
ter starts counting at the current contents in the
timer/event counter and ends at FFH. Once an overflow
occurs, the counter is reloaded from the timer/event
counter preload register, and generates an interrupt re-
quest flag (TF; bit 5 of the INTC ). In the pulse width
measurement mode with the values of the TON and TE
bits equal to 1, after the TMR has received a transient
from low to high (or high to low if the TE bit is ²0²), it will
start counting until the TMR returns to the original level
and resets the TON. The measured result remains in the
timer/event counter even if the activated transient oc-
curs again. In other words, only 1-cycle measurement
can be made until the TON is set. The cycle measure-
ment will re-operate as long as it receives further tran-
sient pulse. In this operation mode, the timer/event
counter begins counting not according to the logic level
but to the transient edges. In the case of counter over-
When the timer/event counter (TMR) is read, the clock is
blocked to avoid errors, as this may results in a counting
error. Blocking of the clock issue should be taken into
account by the programmer. It is strongly recommended
to load a desired value into the TMR register first, before
turning on the related timer/event counter, for proper op-
eration since the initial value of TMR is unknown. Due to
the timer/event scheme, the programmer should pay
special attention on the instruction to enable then dis-
able the timer for the first time, whenever there is a need
to use the timer/event function, to avoid unpredictable
result. After this procedure, the timer/event function can
be operated normally.
The bit0~bit2 of the TMRC can be used to define the
pre-scaling stages of the internal clock sources of the
timer/event counter. The definitions are as shown. The
overflow signal of the timer/event counter can be used
to generate the PFD signal. The timer prescaler is also
used as the PWM counter.
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8-Bit Timer/Event Counter Structure
Rev. 1.40
13
July 12, 2005
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
