CD4541B: About frequency setting

Hi Kenji-san,
It's likely that the oscillator would not be very stable below 1 kHz, but you can always try this out and see how well it works. For this type of circuit, prototyping is the best method for guaranteeing the result you want.

I would recommend using a higher frequency clock (that falls in the 1kHz to 100kHz range), and just adding more bits to the counter to increase the time. For example, a 20 bit counter can be connected to an 1165 Hz oscillator and it will reset at almost exactly 15 minutes (900.064 s). Two of these devices could be configured as 10 bit counters and then cascaded to produce this result.

How accurate do you need the 15 minute timer to be? Using an RC oscillator will likely produce a fairly large variability to the final time (perhaps more than 20% depending on the chosen components).