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HT48R063B Folha de dados(PDF) 39 Page - Holtek Semiconductor Inc |
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HT48R063B Folha de dados(HTML) 39 Page - Holtek Semiconductor Inc |
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39 / 82 page  HT48R063B/064B/065B/066B Rev. 1.20 39 June 3, 2013 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 devices contain from one to three count-up timer of 8-bit capacity. As the timers have three different operating modes, they can be con- figured to operate as a general timer, an external event counter or as a pulse width capture device. The provi- sion 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 source is used when the timer is in the timer mode or in the pulse width capture mode. For some Timer/Event Counters, this internal clock source is first divided by a prescaler, the division ratio of which is conditioned by the Timer Control Regis- ter bits T0PSC0~T0PSC2. For Timer/Event Counter 0, the internal clock source can be either fSYS or the LXT Oscillator, the choice of which is determined by the T0S bit in the TMR0C register. An external clock source is used when the timer is in the event counting mode, the clock source being provided on an external timer pin TCn. Depending upon the con- dition of the TnEG 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 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 regis- ters will be in an unknown condition. Note that if the Timer/Event Counter is in an OFF condition and data is written to its preload register, this data will be immedi- ately written into the actual counter. However, if the counter is enabled and counting, any new data written into the preload data register during this period will re- main 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 control 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. |
Nº de peça semelhante - HT48R063B_13 |
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Descrição semelhante - HT48R063B_13 |
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