PIC16LF720_11 Datasheet, PDF (29/244 Pages) Microchip Technology – 20-Pin Flash Microcontrollers with nanoWatt XLP Technology

3.1 MCLR
The PIC16(L)F720/721 has a noise filter in the MCLR
Reset path. The filter will detect and ignore small
pulses.
It should be noted that a Reset does not drive the
MCLR pin low.
Voltages applied to the pin that exceed its specification
can result in both MCLR Resets and excessive current
beyond the device specification during the ESD event.
For this reason, Microchip recommends that the MCLR
pin no longer be tied directly to VDD. The use of an RC
network, as shown in Figure 3-2, is suggested.
An internal MCLR option is enabled by clearing the
MCLRE bit in the Configuration Word register. When
MCLRE = 0, the Reset signal to the chip is generated
internally. When the MCLRE = 1, the RA3/MCLR pin
becomes an external Reset input. In this mode, the
RA3/MCLR pin has a weak pull-up to VDD. In-Circuit
Serial Programming™ is not affected by selecting the
internal MCLR option.
FIGURE 3-2:
VDD
R1
10 k
RECOMMENDED MCLR
CIRCUIT
PIC^® MCU
MCLR
C1
0.1 F
3.2 Power-on Reset (POR)
The on-chip POR circuit holds the chip in Reset until VDD
has reached a high enough level for proper operation. A
maximum rise time for VDD is required. See
Section 23.0 “Electrical Specifications” for details. If
the BOR is enabled, the maximum rise time specification
does not apply. The BOR circuitry will keep the device in
Reset until VDD reaches VBOR (see Section 3.5
“Brown-Out Reset (BOR)”).
When the device starts normal operation (exits the
Reset condition), device operating parameters (i.e.,
voltage, 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.
For additional information, refer to Application Note
AN607, “Power-up Trouble Shooting” (DS00607).
PIC16(L)F720/721
3.3 Power-up Timer (PWRT)
The Power-up Timer provides a fixed 72 ms (nominal)
time out on power-up only, from POR or Brown-out
Reset. The Power-up Timer operates from the WDT
oscillator. For more information, see Section 7.3
“Internal Clock Modes”. The chip is kept in Reset as
long as PWRT is active. The PWRT delay allows the
VDD to rise to an acceptable level. A Configuration bit,
PWRTE, can disable (if set) or enable (if cleared or pro-
grammed) the Power-up Timer. The Power-up Timer
should be enabled when Brown-out Reset is enabled,
although it is not required.
The Power-up Timer delay will vary from chip-to-chip
and vary due to:
• VDD variation
• Temperature variation
• Process variation
See DC parameters for details (Section 23.0
“Electrical Specifications”).
Note: The Power-up Timer is enabled by the
PWRTE bit in the Configuration Word.
3.4 Watchdog Timer (WDT)
The WDT has the following features:
• Shares an 8-bit prescaler with Timer0
• Time-out period is from 17 ms to 2.2 seconds,
nominal
• Enabled by a Configuration bit
WDT is cleared under certain conditions described in
Table 3-3.
3.4.1 WDT OSCILLATOR
The WDT derives its time base from 31 kHz internal
oscillator.
 2011 Microchip Technology Inc.
Preliminary
DS41430B-page 29