PIC18F45J10 FAMILY
2.2
Power Supply Pins
2.3
Master Clear (MCLR) Pin
The MCLR pin provides two specific device
functions: device Reset, and device programming
and debugging. If programming and debugging are
2.2.1
DECOUPLING CAPACITORS
The use of decoupling capacitors on every pair of
power supply pins, such as VDD, VSS, AVDD and
AVSS, is required.
not required in the end application,
a
direct
connection to VDD may be all that is required. The
addition of other components, to help increase the
application’s resistance to spurious Resets from
Consider the following criteria when using decoupling
capacitors:
voltage sags, may be beneficial.
A
typical
• Value and type of capacitor: A 0.1 μF (100 nF),
10-20V capacitor is recommended. The capacitor
should be a low-ESR device with a resonance
frequency in the range of 200 MHz and higher.
Ceramic capacitors are recommended.
configuration is shown in Figure 2-1. Other circuit
designs may be implemented depending on the
application’s requirements.
During programming and debugging, the resistance
and capacitance that can be added to the pin must
be considered. Device programmers and debuggers
drive the MCLR pin. Consequently, specific voltage
levels (VIH and VIL) and fast signal transitions must
not be adversely affected. Therefore, specific values
of R1 and C1 will need to be adjusted based on the
application and PCB requirements. For example, it is
recommended that the capacitor, C1, be isolated
from the MCLR pin during programming and
debugging operations by using a jumper (Figure 2-2).
The jumper is replaced for normal run-time
operations.
• Placement on the printed circuit board: The
decoupling capacitors should be placed as close
to the pins as possible. It is recommended to
place the capacitors on the same side of the
board as the device. If space is constricted, the
capacitor can be placed on another layer on the
PCB using a via; however, ensure that the trace
length from the pin to the capacitor is no greater
than 0.25 inch (6 mm).
• Handling high-frequency noise: If the board is
experiencing high-frequency noise (upward of
tens of MHz), add a second ceramic type capaci-
tor in parallel to the above described decoupling
capacitor. The value of the second capacitor can
be in the range of 0.01 μF to 0.001 μF. Place this
second capacitor next to each primary decoupling
capacitor. In high-speed circuit designs, consider
implementing a decade pair of capacitances as
close to the power and ground pins as possible
(e.g., 0.1 μF in parallel with 0.001 μF).
Any components associated with the MCLR pin
should be placed within 0.25 inch (6 mm) of the pin.
FIGURE 2-2:
EXAMPLE OF MCLR PIN
CONNECTIONS
VDD
• Maximizing performance: On the board layout
from the power supply circuit, run the power and
return traces to the decoupling capacitors first,
and then to the device pins. This ensures that the
decoupling capacitors are first in the power chain.
Equally important is to keep the trace length
between the capacitor and the power pins to
a minimum, thereby reducing PCB trace
inductance.
R1
R2
MCLR
PIC18FXXJXX
JP
C1
Note 1: R1 ≤ 10 kΩ is recommended. A suggested
starting value is 10 kΩ. Ensure that the
MCLR pin VIH and VIL specifications are met.
2.2.2
TANK CAPACITORS
On boards with power traces running longer than six
inches in length, it is suggested to use a tank capacitor
for integrated circuits including microcontrollers to
supply a local power source. The value of the tank
capacitor should be determined based on the trace
resistance that connects the power supply source to
the device and the maximum current drawn by the
device in the application. In other words, select the tank
capacitor so that it meets the acceptable voltage sag at
the device. Typical values range from 4.7 μF to 47 μF.
2: R2 ≤ 470Ω will limit any current flowing into
MCLR from the external capacitor, C, in the
event of MCLR pin breakdown, due to
Electrostatic Discharge (ESD) or Electrical
Overstress (EOS). Ensure that the MCLR pin
VIH and VIL specifications are met.
DS39682E-page 24
© 2009 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
