Power Supply Pins
The use of decoupling capacitors on every pair of
power supply pins, such as V
Consider the following criteria when using decoupling
• 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.
• 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).
• 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.
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.
Master Clear (MCLR) Pin
functions: device Reset, and device programming
and debugging. If programming and debugging are
not required in the end application, a direct
connection to V
addition of other components, to help increase the
application’s resistance to spurious Resets from
configuration is shown in Figure 2-1. Other circuit
designs may be implemented, depending on the
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
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.
Any components associated with the MCLR pin
should be placed within 0.25 inch (6 mm) of the pin.
Note 1: R1 ≤ 10 kΩ is recommended. A suggested
starting value is 10 kΩ. Ensure that the
MCLR pin V
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
may be all that is required. The
) and fast signal transitions must
EXAMPLE OF MCLR PIN
specifications are met.
specifications are met.
© 2009 Microchip Technology Inc.