HP48 Clock error data results

[Jonathan Busby]

(03-03-2021 04:02 PM)TomC Wrote:  This '72 hour' update detail is very interesting; does this mean that if the machine is awake more often (than every 72 hours), that its clock will be updated more often and hence more accurate?

Well, first, the system time is controlled by TIMER2 which is directly controlled by the crystal oscillator. TIMER2 is a 32-bit signed countdown timer which decrements 8192 times per second. When it underflows ( becomes negative ) the high order bit is set which triggers an interrupt which causes the 48 to wake up and enter the ISR. The time subsystem usually schedules a wakeup for every 72 hours, but, even if more than 72 hours elapses, the system time is still not affected as TIMER2 can be delayed for up to \(2^{31}\) ticks ( 262144 seconds or ~72.88 hours ) without corrupting the system time. My point is, though, that during these wakeups, since the timer service ISR runs with the display off when the 48 wakes up from deep sleep, the system time should be *more* accurate the longer the calculator is turned off ( except for COMA mode which stops TIMER2 and hence nukes the system time ).

Regards,

Jonathan