【導(dǎo)讀】電氣自動(dòng)化專業(yè)畢業(yè)論文--基于單片機(jī)的數(shù)字頻率計(jì)。頻率是最基本的參數(shù)之一并且與許多電參量的測(cè)量方案測(cè)量結(jié)果都有十。分密切的關(guān)系因此頻率的測(cè)量就顯得更為重要測(cè)量頻率的方法有多種其中電子。計(jì)數(shù)器測(cè)量頻率具有精度高使用方便測(cè)量迅速以及便于實(shí)現(xiàn)測(cè)量過(guò)程自動(dòng)化等。優(yōu)點(diǎn)是頻率測(cè)量的重要手段之一電子計(jì)數(shù)器測(cè)頻有兩種方式一是直接測(cè)頻法。即在一定閘門時(shí)間內(nèi)測(cè)量被測(cè)信號(hào)的脈沖個(gè)數(shù)二是間接測(cè)頻法如周期測(cè)頻法。當(dāng)?shù)膫鞲衅骺梢詫?duì)多種物理量進(jìn)行測(cè)試因此它被廣泛應(yīng)用于航天電子測(cè)控等。的頻率的要求而且測(cè)量精度也受到芯片本身極大的限制。12數(shù)字頻率計(jì)概述。頻率測(cè)量所能達(dá)到的精度主要取決于作為標(biāo)準(zhǔn)頻率源的精度以及所使用的測(cè)量。綜觀過(guò)去一二十年的國(guó)內(nèi)頻率計(jì)市場(chǎng)其生產(chǎn)單位有幾十家主要集中在江蘇。需求不斷增長(zhǎng)生產(chǎn)廠商采用新器件和新技術(shù)使頻率計(jì)的便攜性智能性有很大提。4分別輸入方波正弦波鋸齒波和三角波信號(hào)進(jìn)行測(cè)試

　　

【正文】 already loaded into the preload register If the external timer pin is pinshared with other IO pins to ensure that the pin is configured to operate as an event counter input pin two things have to happen The first is to ensure that the T0M0T1M0 and T0M1T1M1 bits place the timerevent counter in the event counting mode the second is to ensure that the port control register configures the pin as an input It should be noted that a timer overflow is one of the interrupt and wakeup sources Configuring the Pulse Width Measurement Mode In this mode the width of external pulses applied to the external timer pin can be measured In the Pulse Width Measurement Mode the timer clock source is supplied by the internal clock For the timer to operate in this mode bits T0M0T1M0 and T0M1T1M1 must both be set high If the T0ET1E bit is low once a high to low transition has been received on the external timer pin the timer will start counting until the external timer pin returns to its original high level At this point the T0ONT1ON bit will be automatically reset to zero and the timer will stop counting If the T0ET1E bit is high the timer will begin counting once a low to high transition has been received on the external timer pin and stop counting when the external timer pin returns to its original low level As before the T0ONT1ON bit will be automatically reset to zero and the timer will stop counting It is important to note that in the Pulse Width Measurement Mode the T0ONT1ON bit is automatically reset to zero when the external control signal on the external timer pin returns to its original level whereas in the other two modes the T0ONT1ON bit can only be reset to zero under program control The residual value in the timer which can now be read by the program therefore represents the length of the pulse received on pin external timer pin As the T0ON T1ON bit has now been reset any further transitions on the external timer pin will be ignored Not until the T0ONT1ON bit is again set high by the program can the timer begin further pulse width measurements In this way single shot pulse measurements can be easily made It should be noted that in this mode the counter is controlled by logical transitions on the external timer pin and not by the logic level As in the case of the other two modes when the counter is full the timer will overflow and generate an internal interrupt signal The counter will also be reset to the value already loaded into the preload register If the external timer pin is pinshared with other IO pins to ensure that the pin is configured to operate as a pulse width measuring input pin two things have to happen The first is to ensure that the T0M0T1M0 and T0M1T1M1 bits place the timerevent counter in the pulse width measuring mode the second is to ensure that the port control register configures the pin as an input It should be noted that a timer overflow is one of the interrupt and wakeup sources Programmable Frequency Divider _ PFD As the HT48RA02HT48CA02 and HT48RA01 HT48CA01 devices do not contain a PFD function note that this section does not apply to these devices For the HT48RA1HT48CA1 HT48RA3HT 48CA3 and HT48RA5HT48CA5 devices each contains a PFD function which is driven by the 8bit counter TMR0By controlling the frequency of the overflow signal from TMR0 the required remote control signal can be generated on the PFD pin Note that a Carrier Generator and associated REM pin is provided on the HT48RA02 HT48CA02 and HT48RA01HT48CA01 devices for the generation of the remote control signal The PFD output is pinshared with the IO pin PB0 The PFD function is selected via a configuration option however if not selected the pin can operate as a normal IO pin The timer overflow signal from the TMR0 8bit counter is the clock source for the PFD circuit The counter is driven by the fSYS clock source or the external timer pin and has an initial value controlled by the value written into the registers The counter will begin to countup from this preload register value until full at which point an overflow signal is generated which is the clock source for the dividebytwo circuit which drives the PFD output The counter will then be automatically reloaded with the preload register value and continue countingup The PFD frequency will therefore be half the frequency of the timer overflow signal Refer to the relevant TimerEvent Counters section for details of its settings and operations The PFD output will only be activated if bit PB0 is set to 1 This output data bit is used as the onoff control bit for the PFD output Note that the PFD output will be low if the PB0 output data bit is cleared to _0_ For the PFD output to function it is also necessary to ensure that the PBC0 bit in the PBC port control register is cleared to _0_ to configure the pin as an output If the PBC0 bit is set to _1_ the PB0 pin will function as an input even if the configuration options have selected the pin to be a PFD output Using this method of frequency generation and if a crystal oscillator is used for the system clockvery precise values of frequency can be generated Prescaler For the 8bit TimerEvent Counter bit 02 of the TMR0C Timer Control Register can be used to define the prescaling stages of the internal clock sources of the TimerEvent Counter The TimerEvent Counter overflow signal can be used to generate PFD signals and as a Timer Interrupt IO Interfacing The TimerEvent Counters when configured to run in the event counter or pulse width measurement mode require the use of the external timer pin for correct operation These external timer pins are pinshared with other Port C input pins which have the option of being connected to pullhigh resistors via a configuration option The 8bit timer can also be setup to drive the pinshared PF