DSPIC30F2011_08 Datasheet, PDF (129/206 Pages) Microchip Technology – High-Performance, 16-bit Digital Signal Controllers

dsPIC30F2011/2012/3012/3013
17.3.1.1 POR with Long Crystal Start-up Time
(with FSCM Enabled)
The oscillator start-up circuitry is not linked to the POR
circuitry. Some crystal circuits (especially low
frequency crystals) will have a relatively long start-up
time. Therefore, one or more of the following conditions
is possible after the POR timer and the PWRT have
expired:
• The oscillator circuit has not begun to oscillate.
• The Oscillator Start-up Timer has not expired (if a
crystal oscillator is used).
• The PLL has not achieved a LOCK (if PLL is
used).
If the FSCM is enabled and one of the above conditions
is true, then a clock failure trap will occur. The device
will automatically switch to the FRC oscillator and the
user can switch to the desired crystal oscillator in the
trap ISR.
17.3.1.2 Operating without FSCM and PWRT
If the FSCM is disabled and the Power-up Timer
(PWRT) is also disabled, then the device will exit rap-
idly from Reset on power-up. If the clock source is
FRC, LPRC, ERC or EC, it will be active immediately.
If the FSCM is disabled and the system clock has not
started, the device will be in a frozen state at the Reset
vector until the system clock starts. From the user’s
perspective, the device will appear to be in Reset until
a system clock is available.
17.3.2 BOR: PROGRAMMABLE
BROWN-OUT RESET
The BOR (Brown-out Reset) module is based on an
internal voltage reference circuit. The main purpose of
the BOR module is to generate a device Reset when a
brown-out condition occurs. Brown-out conditions are
generally caused by glitches on the AC mains
(i.e., missing portions of the AC cycle waveform due to
bad power transmission lines, or voltage sags due to
excessive current draw when a large inductive load is
turned on).
The BOR module allows selection of one of the
following voltage trip points (see Table 20-11):
• 2.6V-2.71V
• 4.1V-4.4V
• 4.58V-4.73V
Note:
The BOR voltage trip points indicated here
are nominal values provided for design
guidance only. Refer to the Electrical
Specifications in the specific device data
sheet for BOR voltage limit specifications.
A BOR will generate a Reset pulse which will reset the
device. The BOR will select the clock source based on
the device Configuration bit values (FOS<2:0> and
FPR<4:0>). Furthermore, if an Oscillator mode is
© 2008 Microchip Technology Inc.
selected, the BOR will activate the Oscillator Start-up
Timer (OST). The system clock is held until OST
expires. If the PLL is used, then the clock will be held
until the LOCK bit (OSCCON<5>) is ‘1’.
Concurrently, the POR time-out (TPOR) and the PWRT
time-out (TPWRT) will be applied before the internal Reset
is released. If TPWRT = 0 and a crystal oscillator is being
used, then a nominal delay of TFSCM = 100 μs is applied.
The total delay in this case is (TPOR + TFSCM).
The BOR Status bit (RCON<1>) will be set to indicate
that a BOR has occurred. The BOR circuit, if enabled,
will continue to operate while in Sleep or Idle modes
and will reset the device should VDD fall below the BOR
threshold voltage.
FIGURE 17-6:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
VDD
D
R
R1
MCLR
C
dsPIC30F
Note 1:
2:
3:
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.
R should be suitably chosen so as to make
sure that the voltage drop across R does not
violate the device’s electrical specifications.
R1 should be suitably chosen so as to limit
any current flowing into MCLR from external
capacitor C, in the event of MCLR/VPP pin
breakdown due to Electrostatic Discharge
(ESD) or Electrical Overstress (EOS).
Note:
Dedicated supervisory devices, such as
the MCP1XX and MCP8XX, may also be
used as an external Power-on Reset
circuit.
DS70139F-page 129