I try to eliminate all unusable combinations and find a step by step procedure in setting the external oscillator, of course not covering all combinations, because that would be too much.
Looking at the datasheet I see that 20MHz external is the highest HS crystal frequency that I can use. I think at higher frequency for higher precision timing. That’s why I choose 20MHz, even if we speak about principle and in reality I do not need the higher precision. Probably I could live also with 4MHz internal. I just need a criteria to decide and I decided for higher precision, lower tolerance, therefore 20MHz external. That is the primary source oscillator that I want to use.
Therefore the first step is to choose at 4) HS or HS+PLL.
Because you mentioned that USB requires PLL to achieve the frequency and timing and I plan to use it later, then seems a must at 4) HS+PLL
Next step is 1), where seems that I have no other option except "USBDIV=2" = "USB Clock Selection: clc src from 96MHz PLL/2”, because I want USB and there is no crystal with so high frequency to provide the USB requirements higher than 20MHz. If I choose “USBDIV=1" = "USB Clocl Selection: clc src from OSC1/OSC2", then the USB is limited at 20MHz.
Therefore at 1) I must choose "USBDIV=2" = "USB Clock Selection: clc src from 96MHz PLL/2”.
Is it not so?
Next step is 3) the “96MHz PLL Prescaler” which must be set at “PLLDIV=5” due to 20MHz/5=4MHz Input Only.
Then we arrive to CPU clock, the last part 2), which can be as you mentioned 48MHz, 32MHz, 24MHz or 16MHz – datasheet page 33 top of table.
Am I right? If yes, then are not too many options left.
Actually that table shows also when I choose HS for 4), without PLL. Then the CPU is frequency is different, lower. What is then about with USB frequency, clock settings?
Looking at the datasheet I see that 20MHz external is the highest HS crystal frequency that I can use. I think at higher frequency for higher precision timing. That’s why I choose 20MHz, even if we speak about principle and in reality I do not need the higher precision. Probably I could live also with 4MHz internal. I just need a criteria to decide and I decided for higher precision, lower tolerance, therefore 20MHz external. That is the primary source oscillator that I want to use.
Therefore the first step is to choose at 4) HS or HS+PLL.
Because you mentioned that USB requires PLL to achieve the frequency and timing and I plan to use it later, then seems a must at 4) HS+PLL
Next step is 1), where seems that I have no other option except "USBDIV=2" = "USB Clock Selection: clc src from 96MHz PLL/2”, because I want USB and there is no crystal with so high frequency to provide the USB requirements higher than 20MHz. If I choose “USBDIV=1" = "USB Clocl Selection: clc src from OSC1/OSC2", then the USB is limited at 20MHz.
Therefore at 1) I must choose "USBDIV=2" = "USB Clock Selection: clc src from 96MHz PLL/2”.
Is it not so?
Next step is 3) the “96MHz PLL Prescaler” which must be set at “PLLDIV=5” due to 20MHz/5=4MHz Input Only.
Then we arrive to CPU clock, the last part 2), which can be as you mentioned 48MHz, 32MHz, 24MHz or 16MHz – datasheet page 33 top of table.
Am I right? If yes, then are not too many options left.
Actually that table shows also when I choose HS for 4), without PLL. Then the CPU is frequency is different, lower. What is then about with USB frequency, clock settings?