All inputs of the C16x family have Schmitt-Trigger input characteristics. These Schmitt-Triggers are intended to always provide proper internal low and high levels, even if anundefined voltage level (between TTL-VIL and TTL-VIH) is externally applied to the pin.The hysteresis of these inputs, however, is very small, and can not be properly used in anapplication to suppress signal noise, and to shape slow rising/falling input transitions.Thus, it must be taken care that rising/falling input signals pass the undefined area of theTTL-specification between VIL and VIH with a sufficient rise/fall time, as generally usualand specified for TTL components (e.g. 74LS series: gates 1V/us, clock inputs 20V/us).The effect of the implemented Schmitt-Trigger is that even if the input signal remains inthe undefined area, well defined low/high levels are generated internally. Note that allinput signals are evaluated at specific sample points (depending on the input and theperipheral function connected to it), at that signal transitions are detected if twoconsecutive samples show different levels. Thus, only the current level of an input signalat these sample points is relevant, that means, the necessary rise/fall times of the inputsignal is only dependant on the sample rate, that is the distance in time between twoconsecutive evaluation time points. If an input signal, for instance, is sampled throughsoftware every 10us, it is irrelevant, which input level would be seen between thesamples. Thus, it would be allowable for the signal to take 10us to pass through theundefined area. Due to the sample rate of 10us, it is assured that only one sample canoccur while the signal is within the undefined area, and no incorrect transition will bedetected. For inputs which are connected to a peripheral function, e.g. capture inputs, thesample rate is determined by the clock cycle of the peripheral unit. In the case of theCAPCOM unit this means a sample rate of 400ns @ 20MHz CPU clock. This requiresinput signals to pass through the undefined area within these 400ns in order to avoidmultiple capture events.For input signals, which do not provide the required rise/fall times, external circuitry mustbe used to shape the signal transitions.In the attached diagram, the effect of the sample rate is shown. The numbers 1 to 5 in thediagram represent possible sample points. Waveform a) shows the result if the inputsignal transition time through the undefined TTL-level area is less than the time distancebetween the sample points (sampling at 1, 2, 3, and 4). Waveform b) can be the result ifthe sampling is performed more than once within the undefined area (sampling at 1, 2, 5,3, and 4).Sample points:1. Evaluation of the signal clearly results in a low level2. Either a low or a high level can be sampled here. If low is sampled, no transition willbe detected. If the sample results in a high level, a transition is detected, and anappropriate action (e.g. capture) might take place.3. Evaluation here clearly results in a high level. If the previous sample 2) had alreadydetected a high, there is no change. If the previous sample 2) showed a low, atransition from low to high is detected now.
標簽：
Signal
Input
Fall
Rise
上傳時間：
2013-10-23
上傳用戶：copu
摘 要:用一種新的思路和方法,先計算低通、再計算高通濾波器的有關參數(shù),然后組合成帶通濾波器.關鍵詞:濾波器;參數(shù);新思路中圖分類號: TN713. 5 文獻識別碼:B 文章編號:1008 - 1666 (1999) 04 - 0089 - 03A New Consideration of the Band Filter’s CalculationGuo Wencheng( S hao Yang B usiness and Technology school , S haoyang , Hunan ,422000 )Abstract :This essay deals with a new method of calculating the band filters - first calculatingthe relevant parameters of low - pass filters ,then calculating the ones of high - pass filters.Key words :filter ; parameters ;new considercation八十年代后,信息產(chǎn)業(yè)得到了迅猛發(fā)展. 帶通濾波器在微波通信、廣播電視和精密儀器設備中得到了廣泛應用. 帶通濾波器性能的優(yōu)劣,對提高接收機信噪比,防止鄰近信道干擾,提高設備的技術指標,有著十分重要的意義.我在長期的教學實踐中,用切比雪夫型方法設計、計算出寬帶濾波器集中參數(shù)元件的數(shù)據(jù). 該濾波器可運用在檢測微波頻率的儀器和其他設備中. 再將其思路和計算方法介紹給大家,供參考.
標簽：
帶通濾波器設計
計算
上傳時間：
2014-12-28
上傳用戶：Yukiseop
