Specifications
mikroElektronika | Free Online Book | PIC Microcontrollers | Chapter 4: Timers
Fig. 4-6 Timer TMR1 Overview
Timer TMR1 Prescaler
Timer TMR1 has a completely separate prescaler which allows 1, 2, 4 or 8 divisions of the clock input. The prescaler is not
directly readable or writable. However, the prescaler counter is automatically cleared upon write to the TMR1H or TMR1L
register.
Timer TMR1 Oscillator
RC0/T1OSO and RC1/T1OSI pins are used to register pulses coming from peripheral electronics, but they also have an
additional function. As seen in figure 4-7, they are simultaneously configured as both input (pin RC1) and output (pin RC0)
of the additional LP quartz oscillator (low power).
This additional circuit is primarily designed for operating at low frequencies (up to 200 KHz), more precisely, for using the
32,768 KHz quartz crystal. Such crystals are used in quartz watches because it is easy to obtain one-second-long pulses by
simply dividing this frequency.
Since this oscillator does not depend on internal clocking, it can operate even in sleep mode. It is enabled by setting the
T1OSCEN control bit of the T1CON register. The user must provide a software time delay (a few milliseconds) to ensure
proper oscillator start-up.
Table below shows the recommended values of capacitors to suit the quartz oscillator.
These values do not have to be exact. However, the general rule is: the higher the
capacitor's capacity the higher the stability, which, at the same time, prolongs the time
needed for the oscillator stability.
Oscillator Frequency C1 C2
LP
32 kHz 33 pF 33 pF
100 kHz 15 pF 15 pF
200 kHz 15 pF 15 pF
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