Clock Trees

Many peripheral problems do not look like clock problems at first.

UART baud rate is slightly wrong. SPI fails when the rate is raised. I2C occasionally times out. A timer period drifts. ADC sampling does not match the expected rate. The CPU runs at a high frequency, but peripheral register access is still slow. A more subtle case: everything works before sleep, but the first peripheral access after wakeup fails, or a UART works in the bootloader and then stops working once the operating system takes over.