是否可以用它控制用户的周期或频率来​​构建时钟？

米兰，
通过使用驱动DDFS的晶体振荡器和改变数字控制字，可以使用直接数字频率合成器（DDFS）作为数控（NCO）。
除了DDFS之外，还有其他种类的NCO。
谷歌“数控振荡器”以及“直接数字频率合成”。
可以在DFS或PLL的反馈路径中选择各种M和D值，因此可以通过固定的小步骤改变频率：32 / 32,31 / 32,30 / 32,29 / 32,28 / 32,27
/ 32，...... 1/32。
可能存在一些范围问题，因为最小和最大频率都会使这很困难（但输出必须在32/32和1/32！的规格范围内）。
可以使用电位计来选择输入到A / D转换器（如V5或V6上的系统监视器）的电压（分压器），以产生变化的数字控制字。
或者，只要按住按钮，就可以使用按钮来“递增”或“递减”频率范围。
I DIP开关可用于输入所需的频率，或键盘或键盘。
Austin Lesea主要工程师Xilinx San Jose



以下为原文

milan,
 
One can use a direct digital frequency synthesizer (DDFS) as a numerically controlled (NCO) by using a crystal oscillator driving the DDFS, and varying the digital control word.

There are other sorts of NCO, besides the DDFS.
 
Google "numerically controlled oscillator" and also "direct digital frequency synthesis."
 
One can choose various M and D values in the feedback path of the DFS or PLL, so one can vary the frequency in small fixed steps:  32/32, 31/32, 30/32, 29/32, 28/32, 27/32, .... 1/32.  There may be some range issues, for both minimum and maximum frequencies that make this difficult, however (the output must be within specification for both 32/32, and 1/32!).
 
One may use a potentiometer to select a voltage (a voltage divider) input to an A/D converter (like the system monitor on V5 or V6) to create a varying digital control word.
 
Or, one may just use push buttons to "increment" or "decrement" the frequency range as long as the push button is held down.  I DIP switch could be used to input the frequency desired, or a keypad, or keyboard.
 
Austin Lesea
Principal Engineer
Xilinx San Jose