
PIC18F1220/1320
DS39605C-page 34
2004 Microchip Technology Inc.
4.1
Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when
VDD rise is detected. To take advantage of the POR
circuitry, just tie the MCLR pin through a resistor (1k to
10 k
) to VDD. This will eliminate external RC compo-
nents usually needed to create a Power-on Reset
delay. A minimum rise rate for VDD is specified
When the device starts normal operation (i.e., exits the
Reset condition), device operating parameters (volt-
age, frequency, temperature, etc.) must be met to
ensure operation. If these conditions are not met, the
device must be held in Reset until the operating
conditions are met.
FIGURE 4-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
4.2
Power-up Timer (PWRT)
The Power-up Timer (PWRT) of the PIC18F1220/1320 is
an 11-bit counter, which uses the INTRC source as the
clock input. This yields a count of 2048 x 32
s= 65.6 ms.
While the PWRT is counting, the device is held in Reset.
The power-up time delay will vary from chip-to-chip due
to VDD, temperature and process variation. See DC
parameter
33 for details.
The PWRT is enabled by clearing configuration bit,
PWRTEN.
4.3
Oscillator Start-up Timer (OST)
The Oscillator Start-up Timer (OST) provides a 1024
oscillator cycle (from OSC1 input) delay after the
PWRT delay is over (parameter
33). This ensures that
the crystal oscillator or resonator has started and
stabilized.
The OST time-out is invoked only for XT, LP, HS and
HSPLL modes and only on Power-on Reset, or on exit
from most low-power modes.
4.4
PLL Lock Time-out
With the PLL enabled in its PLL mode, the time-out
sequence following a Power-on Reset is slightly
different from other oscillator modes. A portion of the
Power-up Timer is used to provide a fixed time-out that
is sufficient for the PLL to lock to the main oscillator fre-
quency. This PLL lock time-out (TPLL) is typically 2 ms
and follows the Oscillator Start-up Time-out.
4.5
Brown-out Reset (BOR)
A configuration bit, BOR, can disable (if clear/
programmed), or enable (if set) the Brown-out Reset
circuitry. If VDD falls below VBOR (parameter D005) for
greater than TBOR (parameter 35), the brown-out situa- tion will reset the chip. A Reset may not occur if VDD
falls below VBOR for less than TBOR. The chip will
remain in Brown-out Reset until VDD rises above VBOR.
If the Power-up Timer is enabled, it will be invoked after
VDD rises above VBOR; it then will keep the chip in
Reset
for
an
additional
time
delay,
TPWRT
(parameter 33). If VDD drops below VBOR while the Power-up Timer is running, the chip will go back into a
Brown-out Reset and the Power-up Timer will be initial-
ized. Once VDD rises above VBOR, the Power-up Timer
will execute the additional time delay. Enabling BOR
Reset does not automatically enable the PWRT.
4.6
Time-out Sequence
On power-up, the time-out sequence is as follows:
First, after the POR pulse has cleared, PWRT time-out
is invoked (if enabled). Then, the OST is activated. The
total time-out will vary based on oscillator configuration
and the status of the PWRT. For example, in RC mode
with the PWRT disabled, there will be no time-out at all.
Since the time-outs occur from the POR pulse, if MCLR
is kept low long enough, all time-outs will expire. Bring-
ing MCLR high will begin execution immediately
synchronize more than one PIC18FXXXX device
operating in parallel.
Table 4-2 shows the Reset conditions for some Special
Function Registers, while
Table 4-3 shows the Reset
conditions for all the registers.
Note
1: External Power-on Reset circuit is required
only if the VDD power-up slope is too slow.
The diode D helps discharge the capacitor
quickly when VDD powers down.
2: R < 40 k
is recommended to make sure that
the voltage drop across R does not violate
the device’s electrical specification.
3: R1
≥ 1 k will limit any current flowing into
MCLR from external capacitor C, in the event
of MCLR/VPP pin breakdown due to Electro-
static
Discharge
(ESD)
or
Electrical
Overstress (EOS).
C
R1
R
D
VDD
MCLR
PIC18FXXXX
VDD