HT45R22E Holtek Semiconductor, HT45R22E Datasheet - Page 34

• Manufacturer Part Number: HT45R22E
• Description: Remote Type 8-bit OTP MCU
• Manufacturer: Holtek Semiconductor

Timer/Event Counters
The provision of timers form an important part of any
microcontroller, giving the designer a means of carrying
out time related functions. The device contains two
count-up timers of 8-bit capacity. As the timers have
three different operating modes, they can be configured
to operate as a general timer, an external event counter
or as a pulse width capture device. The provision of an
internal prescaler to the clock circuitry on gives added
range to the timers.
There are two types of registers related to the
Timer/Event Counters. The first is the register that con-
tains the actual value of the timer and into which an ini-
tial 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 deter-
mines 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.
Configuring the Timer/Event Counter Input Clock
Source
The Timer/Event Counter clock source can originate
from various sources, an internal clock or an external
pin. The internal clock source is used when the timer is
in the timer mode or in the pulse width capture mode.
For Timer/Event Counter 0, this internal clock source is
first divided by a prescaler, the division ratio of which is
conditioned by the Timer Control Register bits
T0PSC0~T0PSC2. For Timer/Event Counter 0, the in-
ternal clock source can be either f
tor, the choice of which is determined by the T0S bit in
the TMR0C register.
An external clock source is used when the Timer/Event
Counter n is in the event counting mode, the clock
source being provided on an external timer pin TCn. De-
pending upon the condition of the TnE bit, each high to
low, or low to high transition on the external timer pin will
increment the counter by one.
Timer Registers - TMR0, TMR1
The timer registers are special function registers located
in the Special Purpose Data Memory and is the place
where the actual timer value is stored. These registers
are known as TMR0 and TMR1. The value in the timer
registers increases by one each time an internal clock
pulse is received or an external transition occurs on the
external timer pin. The timer will count from the initial
Rev. 1.00
SYS
or the LXT Oscilla-
34
value loaded by the preload register to the full count of
FFH at which point the timer overflows and an internal
interrupt signal is generated. The timer value will then
be reset with the initial preload register value and con-
tinue counting. Note that to achieve a maximum full
range count of FFH, the preload register must first be
cleared to all zeros. It should be noted that after
power-on, the preload registers will be in an unknown
condition.
Note that if the Timer/Event Counter is in an OFF condi-
tion and data is written to its preload register, this data
will be immediately written into the actual counter. How-
ever, if the counter is enabled and counting, any new
data written into the preload data register during this pe-
riod will remain in the preload register and will only be
written into the actual counter the next time an overflow
occurs.
Timer Control Registers - TMR0C, TMR1C
The flexible features of the Holtek microcontroller
Timer/Event Counters enable them to operate in three
different modes, the options of which are determined by
the contents of their respective control register.
The Timer Control Register is known as TMRnC. It is the
Timer Control Register together with its corresponding
Timer Register that controls the full operation of the
Timer/Event Counter. Before the timer can be used, it is
essential that the Timer Control Register is fully pro-
grammed with the right data to ensure its correct opera-
tion, a process that is normally carried out during
program initialisation.
To choose which of the three modes the timer is to oper-
ate in, either in the timer mode, the event counting mode
or the pulse width capture mode, bits 7 and 6 of the
Timer Control Register, which are known as the bit pair
TnM1/TnM0, must be set to the required logic levels.
The timer-on bit, which is bit 4 of the Timer Control Reg-
ister and known as TnON, provides the basic on/off con-
trol of the respective timer. Setting the bit high allows the
counter to run, clearing the bit stops the counter. Bits
0~2 of the Timer Control Register determine the division
ratio of the input clock prescaler. The prescaler bit set-
tings have no effect if an external clock source is used. If
the timer is in the event count or pulse width capture
mode, the active transition edge level type is selected by
the logic level of bit 3 of the Timer Control Register
which is known as TnEG. The TnS bit selects the inter-
nal clock source if used.
HT45R22E
March 17, 2010
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