I am trying to cut down the power consumption for an ATMega32u4.
The datasheet says, for USB 2.0 it needs an external clock, which it uses to generate a 48 MHz clock by multiplying the external clock up.
But it doesn't say anything about what frequency that external clock should have.
What is the minimum clock frequency that I can use while still retaining USB 2.0 capabilities?
Does the CLKDIV8 fuse affect that in any way?
The USB module needs a 48MHz clock. This is generated by a PLL circuit. The input frequency for the PLL typically is 8 MHz or 16 MHz, but other relations can be configured. You don't need an external clock generator but the allowed frequency deviation of USB systems require that the crystal oscillator mode is used.
To reduce power consumption you setup an 8 MHz crystal oscillator and use sleep modes. A USB connect event can be configured to wake up the MCU, however the USB setup code should run again for proper operation.
I like to use the watchdog timer to create a wake up interval where the control processing is performed with high speed and let the MCU sleep the remaining time.
As I understand, the CLKDIV8 fuse cannot be used in this context.
