The high speed mode is enabled as default and the fast peripheral clock is 64 MHz, but the low speed mode can be set by writing the LSM bit to one. Then the fast peripheral clock is scaled down to 32 MHz. The low speed mode must be set, if the supply voltage is below 2.7 volts (!!!), because the Timer/Counter1 is not running fast enough on low voltage levels. It is highly recommended that Timer/Counter1 is stopped whenever the LSM bit is changed."