5509A外挂12M的晶振那么USB默认就是48M的频率,但是我现在必须要外部挂20M的晶振,还是想保持USB的48M的频率需要改哪里的配置啊?
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5509A外挂12M的晶振那么USB默认就是48M的频率,但是我现在必须要外部挂20M的晶振,还是想保持USB的48M的频率需要改哪里的配置啊?
如果要用USB的话,CLKIN必须是48MHz的分频,20MHz晶振没法用。数据手册里对CLKIN有说明。
NOTE: The USB module requires a 48 MHz clock. Since this input clock
is used by both the CPU PLL and the USB module PLL, it must
be a factor of 48 MHz in order for the programmable PLL to
produce the required 48 MHz USB module clock.
http://www.ti.com/lit/ds/symlink/tms320vc5509a.pdf