HT46R01A
Programming Considerations
count-up timer of 8-bit capacity. As the timer has three
different operating modes, it can be configured to oper-
ate as a general timer, an external event counter or as a
pulse width measurement device. The provision of an
internal prescaler to the clock circuitry gives added
range to the timer.
Within the user program, one of the first things to con-
sider is port initialisation. After a reset, the PA data regis-
ter and PAC port control register will be set high. This
means that all I/O pins will default to an input state, the
level of which depends on the other connected circuitry
and whether pull-high options have been selected. If the
PAC port control register, is then programmed to setup
some pins as outputs, these output pins will have an ini-
tial high output value unless the associated PA port data
register is first programmed. Selecting which pins are in-
puts and which are outputs can be achieved byte-wide
by loading the correct value into the port control register
or by programming individual bits in the port control reg-
ister using the ²SET [m].i² and ²CLR [m].i² instructions.
Note that when using these bit control instructions, a
read-modify-write operation takes place. The
microcontroller must first read in the data on the entire
port, modify it to the required new bit values and then re-
write this data back to the output ports.
There are two types of registers related to the
Timer/Event Counter. The first is the register that con-
tains the actual value of the Timer/Event Counter and
into which an initial value can be preloaded. Reading
from this register retrieves the contents of the
Timer/Event Counter. The second type of associated
register is the Timer Control Register which defines the
timer options and determines how the timer is to be
used. The device can have the timer clock configured to
come from the internal clock source. In addition, the
timer clock source can also be configured to come from
an external timer pin.
An external clock source is used when the timer is in the
event counting mode, the clock source being provided
on the external timer pin, known as TMR0. These exter-
nal timer pins are pin-shared with other I/O pins. De-
pending upon the condition of the T0E bit in the
corresponding Timer Control Register, each high to low,
or low to high transition on the external timer input pin
will increment the counter by one.
T
1
T
2
T
3
T
4
T
1
T
2
T
3
T
4
S
y
s
t
e
m
C
l
o
c
k
P
o
r
t
D
a
t
a
W
r
i
t
e
t
o
P
o
r
t
R
e
a
d
f
r
o
m
P
o
r
t
Read/Write Timing
Configuring the Timer/Event Counter Input Clock
Source
Pins PA0 to PA7 each have a wake-up functions, se-
lected via the PAWK register. When the device is in the
Power Down Mode, various methods are available to
wake the device up. One of these is a high to low transi-
tion of any of the these pins. Single or multiple pins on
Port A can be setup to have this function.
The internal timer¢s clock can originate from various
sources, depending upon which device and which timer
is chosen. The system clock input timer source is used
when the timer is in the timer mode or in the pulse width
measurement mode. This system clock timer source is
first divided by a prescaler, the division ratio of which is
conditioned by the Timer Control Register bits
T0PSC0~T0PSC2.
Timer/Event Counter
The provision of timers form an important part of any
microcontroller, giving the designer a means of carrying
out time related functions. This device contains one
D
a
t
a
B
u
s
R
e
l
o
a
d
P
r
e
l
o
a
d
R
e
g
i
s
t
e
r
T
0
P
S
C
2
~
T
0
P
S
C
0
T
0
M
1
T
0
M
0
(
1
/
2
~
1
/
2
5
6
)
M
f
S Y S
8
-
s
t
a
g
e
p
r
e
s
c
a
l
e
r
U
O
v
e
r
f
l
o
w
T
i
m
e
r
/
E
v
e
n
t
C
o
u
n
t
e
r
T
i
m
e
r
/
E
v
e
n
t
C
o
u
n
t
e
r
R
T
C
O
s
c
i
l
l
a
t
o
r
X
t
P
o
I
n
t
e
r
r
u
B
p
t
M
o
d
e
C
o
n
t
r
o
l
T
0
O
N
T
0
S
¸
2
F
D
o
r
u
z
z
e
r
T
M
R
0
T
0
E
8-bit Timer/Event Counter Structure
Rev. 1.10
17
August 13, 2008
HOLTIC [ HOLT INTEGRATED CIRCUITS ]
