PIC18F25J50 MICROCHIP [Microchip Technology], PIC18F25J50 Datasheet - Page 205
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PIC18F25J50
Manufacturer Part Number
PIC18F25J50
Description
28/44-Pin, Low-Power, High-Performance USB Microcontrollers with nanoWatt XLP Technology
Manufacturer
MICROCHIP [Microchip Technology]
Datasheet
1.PIC18F25J50.pdf
(562 pages)
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13.4
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
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
for the Timer1 oscillator.
The user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator.
FIGURE 13-2:
2011 Microchip Technology Inc.
Note:
13-2.
Timer1 16-Bit Read/Write Mode
Timer1 Oscillator
12 pF
12 pF
C1
C2
See the Notes with
information about capacitor selection.
Table 13-2
32.768 kHz
XTAL
EXTERNAL COMPONENTS
FOR THE TIMER1 LP
OSCILLATOR
provides the capacitor selection
T1OSI
T1OSO
Table 13-2
PIC18F46J50
for additional
PIC18F46J50 FAMILY
TABLE 13-2:
The Timer1 crystal oscillator drive level is determined
based on the LPT1OSC (CONFIG2L<4>) Configura-
tion bit. The Higher Drive Level mode, LPT1OSC = 1,
is intended to drive a wide variety of 32.768 kHz
crystals with a variety of load capacitance (C
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 C
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
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.
Oscillator
Note 1: Microchip suggests these values as a
Type
LP
2: Higher capacitance increases the stabil-
3: Since each resonator/crystal has its own
4: Capacitor values are for design guidance
5: Incorrect capacitance value may result in
starting point in validating the oscillator
circuit.
ity of the oscillator but also increases the
start-up time.
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate
components.
only. Values listed would be typical of a
C
LPT1OSC = 1.
a frequency not meeting the crystal
manufacturer’s tolerance specification.
L
= 10
32 kHz
Freq.
CAPACITOR SELECTION FOR
THE TIMER
OSCILLATOR
pF
values
rated
12 pF
C1
(2,3,4,5)
DS39931D-page 205
(1)
Figure
crystal
of
12 pF
L
13-2. See
external
) ratings.
C2
L
when
(1)
(load
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