PIC18F47J53 Datasheet, PDF (213/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

PIC18F47J53 FAMILY
13.4 Timer1 16-Bit Read/Write Mode
Timer1 can be configured for 16-bit reads and writes.
When the RD16 control bit (T1CON<1>) is set, the
address for TMR1H is mapped to a buffer register for
the high byte of Timer1. A read from TMR1L loads the
contents of the high byte of Timer1 into the Timer1 High
Byte Buffer register. This provides the user with the
ability to accurately read all 16 bits of Timer1 without
having to determine whether a read of the high byte,
followed by a read of the low byte, has become invalid
due to a rollover between reads.
A write to the high byte of Timer1 must also take place
through the TMR1H Buffer register. The Timer1 high
byte is updated with the contents of TMR1H when a
write occurs to TMR1L. This allows a user to write all
16 bits to both the high and low bytes of Timer1 at once.
The high byte of Timer1 is not directly readable or
writable in this mode. All reads and writes must take
place through the Timer1 High Byte Buffer register.
Writes to TMR1H do not clear the Timer1 prescaler.
The prescaler is only cleared on writes to TMR1L.
13.5 Timer1 Oscillator
An on-chip crystal oscillator circuit is incorporated
between pins, T1OSI (input) and T1OSO (amplifier
output). It is enabled by setting the Timer1 Oscillator
Enable bit, T1OSCEN (T1CON<3>). The oscillator is a
low-power circuit rated for 32 kHz crystals. It will
continue to run during all power-managed modes. The
circuit for a typical LP oscillator is depicted in
Figure 13-2. Table 13-2 provides the capacitor selection
for the Timer1 oscillator.
The user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator.
FIGURE 13-2:
C1
12 pF
EXTERNAL COMPONENTS
FOR THE TIMER1 LP
OSCILLATOR
PIC18F47J53
T1OSI
XTAL
32.768 kHz
C2
12 pF
T1OSO
Note:
See the Notes with Table 13-2 for additional
information about capacitor selection.
TABLE 13-2:
CAPACITOR SELECTION FOR
THE TIMER
OSCILLATOR^(2,3,4,5)
Oscillator
Type
LP
Freq.
32 kHz
C1
12 pF⁽¹⁾
C2
12 pF⁽¹⁾
Note 1: Microchip suggests these values as a
starting point in validating the oscillator
circuit.
2: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
4: Capacitor values are for design guidance
only. Values listed would be typical of a
CL = 10 pF rated crystal when
SOSCSEL<1:0> = 11.
5: Incorrect capacitance value may result in
a frequency not meeting the crystal
manufacturer’s tolerance specification.
The Timer1 crystal oscillator drive level is determined
based on the SOSCSEL (CONFIG2L<4:3>) Configura-
tion bits. The Higher Drive Level mode
(SOSCSEL<1:0> = 11) is intended to drive a wide vari-
ety of 32.768 kHz crystals with a variety of load capac-
itance (CL) ratings.
The Lower Drive Level mode is highly optimized for
extremely low-power consumption. It is not intended to
drive all types of 32.768 kHz crystals. In the Low Drive
Level mode, the crystal oscillator circuit may not work
correctly if excessively large discrete capacitors are
placed on the T1OSI and T1OSO pins. This mode is
only designed to work with discrete capacitances of
approximately 3 pF-10 pF on each pin.
Crystal manufacturers usually specify a CL (load
capacitance) rating for their crystals. This value is
related to, but not necessarily the same as, the values
that should be used for C1 and C2 in Figure 13-2. See
the crystal manufacturer’s applications information for
more details on how to select the optimum C1 and C2
for a given crystal. The optimum value depends in part
on the amount of parasitic capacitance in the circuit,
which is often unknown. Therefore, after values have
been selected, it is highly recommended that thorough
testing and validation of the oscillator be performed.
 2010 Microchip Technology Inc.
Preliminary
DS39964B-page 213