Datasheet

© 2007-2012 Microchip Technology Inc. DS70293G-page 65
PIC24HJ32GP302/304, PIC24HJ64GPX02/X04 AND PIC24HJ128GPX02/X04
6.4 Power-on Reset (POR)
A Power-on Reset (POR) circuit ensures the device is
reset from power-on. The POR circuit is active until
V
DD crosses the VPOR threshold and the delay TPOR
has elapsed. The delay TPOR ensures the internal
device bias circuits become stable.
The device supply voltage characteristics must meet
the specified starting voltage and rise rate
requirements to generate the POR. Refer to
Section 28.0 “Electrical Characteristics” for details.
The POR status bit (POR) in the Reset Control register
(RCON<0>) is set to indicate the Power-on Reset.
6.4.1 Brown-out Reset (BOR) and
Power-up timer (PWRT)
The on-chip regulator has a Brown-out Reset (BOR)
circuit that resets the device when the VDD is too low
(V
DD < VBOR) for proper device operation. The BOR cir-
cuit keeps the device in Reset until VDD crosses VBOR
threshold and the delay TBOR has elapsed. The delay
T
BOR ensures the voltage regulator output becomes
stable.
The Brown-out Reset status bit (BOR) in the Reset
Control register (RCON<1>) is set to indicate the BOR.
The device will not run at full speed after a BOR as the
V
DD should rise to acceptable levels for full-speed
operation. The PWRT provides power-up time delay
(TPWRT) to ensure that the system power supplies have
stabilized at the appropriate levels for full-speed
operation before the SYSRST
is released.
The power-up timer delay (T
PWRT) is programmed by
the Power-on Reset Timer Value Select bits
(FPWRT<2:0>) in the POR Configuration register
(FPOR<2:0>), which provides eight settings (from 0 ms
to 128 ms). Refer to Section 25.0 “Special Features”
for further details.
Figure 6-3 shows the typical brown-out scenarios. The
reset delay (T
BOR + TPWRT) is initiated each time VDD
rises above the VBOR trip point
TABLE 6-2: OSCILLATOR DELAY
Symbol Parameter Value
VPOR POR threshold 1.8V nominal
T
POR POR extension time 30 μs maximum
V
BOR BOR threshold 2.5V nominal
T
BOR BOR extension time 100 μs maximum
T
PWRT Programmable power-up time delay 0-128 ms nominal
TFSCM Fail-Safe Clock Monitor Delay 900 μs maximum
Note: When the device exits the Reset
condition (begins normal operation), the
device operating parameters (voltage,
frequency, temperature, etc.) must be
within their operating ranges, otherwise
the device may not function correctly.
The user application must ensure that
the delay between the time power is
first applied, and the time SYSRST
becomes inactive, is long enough to get
all operating parameters within
specification.