PIC18F252如何使用外部RA4/T0CKL避免1/4 DeVIDER

哇，真是个老古董。15年前开始使用PIC18S时，我使用了新的设备。最好的解决方案是升级到一个能在64 MHz运行的新芯片，并使用信号测量定时器获得64 MZZ分辨率。我怀疑你是在一个教育环境中，但是你必须使用你所得到的。你可以用40MHz的振荡器在40MHz上运行PIC18F252，并把你的分辨率提高一倍。外部频率来自你连接它的任何地方。如果你想在T0CKI上20MHz，那么你需要从某个东西得到一个20MHz的时钟。也许用振荡器代替20MHz晶体，同时从它同时驱动OSC1和T0CKI。请注意，图10-2和表22-8显示T0CKI不能比指令时钟快（20MHz／4），除非你使用预分频器来破坏你的目的。IO引脚到T0CKI，两者通过电阻输入时钟。这个想法是三态IO通常允许外部时钟通过T0CKI。当定时周期完成时，驱动IO低以覆盖外部时钟并停止时钟。然后脉冲它高/低，并计数有多少脉冲来翻转TMR0。它告诉你预分频器的值。我相信这是唯一的方法来获得更高的定时器分辨率与这个古老的设备。然后你可以在50MHz运行TMR0。



      以下为原文

    Wow, what an old chip. I was using newer devices when I started with PIC18s 15 years ago.
 
The best solution would be to upgrade to a newer chip that can run at 64Mhz, and use the Signal Measurement Timer to get 64MZz resolution. I suspect that you are in an education environment, however, where you have to use what you are given.
 
You could run the PIC18F252 at 40MHz with a 40MHz oscillator and double your resolution.
 
The external frequency comes from wherever you connect it. If you want 20MHz on T0CKI, then you need to have a 20MHz clock from something. Maybe replace the 20MHz crystal with an oscillator and drive both OSC1 and T0CKI from it.
 
Note that figure 10-2 and table 22-8 shows T0CKI can't be faster than the instruction clock (20MHz/4) unless you use the prescaler which defeats your purpose.
 
There is a trick I saw somewhere to connect another IO pin to T0CKI, and both to the input clock through a resistor. The idea was to tri-state the IO normally which allows the external clock through to T0CKI. Drive the IO low when the timing period is done to override the external clock and stop clocking. Then pulse it high/low and count how many pulses it took to rollover TMR0. That tells you the value in the prescaler. I believe this is the only way to get higher timer resolution with this ancient device. Then you can run Tmr0 at 50MHz.