7.8 Event counter mode
Event counter mode is selected by bits 4 and 5 which are logic 10 in the control and status register. The event counter mode is used to count pulses externally applied to the oscillator input (OSCO left open-circuit). The event counter stores up to 6 digits of data, which are stored as 6 hexadecimal values located in the registers 1h, 2h, and 3h. Therefore, up to 1 million events may be recorded. An event counter alarm occurs when the event counter registers match the value programmed in the registers 9h, Ah, and Bh, and the event alarm is enabled (bits 4 and 5 which are logic 01 in the alarm control register). In this event, the alarm flag (bit 1 of the control and status register) is set. The inverted value of this flag can be transferred to the interrupt pin (pin 7) by setting the alarm interrupt enable in the alarm control register. In this mode, the timer (location 07h) increments once for every one, one hundred, ten
thousand, or 1 million events, depending on the value programmed in bits 0, 1 and 2 of the alarm control register. In all other events, the timer functions are as in the clock mode.

У микросхемы PCF8583 есть дополнительные ячейки памяти, доступные для для записи/чтения,которые можно попытаться использовать для расширения возможностей счетчика. И 254 млн. это не предел...