PIC24FJ64GA705 Datasheet, PDF (30/412 Pages) –

PIC24FJ256GA705 FAMILY
2.2 Power Supply Pins
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.
Consider the following criteria when using decoupling
capacitors:
• Value and type of capacitor: A 0.1 F (100 nF),
25V-50V capacitor is recommended. The
capacitor should be a low-ESR device with a
self-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 F to 0.001 F. 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 F in parallel with 0.001 F).
• 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.
2.2.2 BULK CAPACITORS
On boards with power traces running longer than six
inches in length, it is suggested to use a bulk
capacitance of 10 µF or greater located near the MCU.
The value of the 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. Typical values range
from 10 µF to 47 µF. The capacitor should be ceramic
and have a voltage rating of 25V or more to reduce DC
bias effects (see Section 2.4.1 “Considerations for
Ceramic Capacitors”).
2.3 Master Clear (MCLR) Pin
The MCLR pin provides two specific device functions:
device Reset, and device programming and debug-
ging. If programming and debugging are 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 voltage sags, may
be beneficial. A typical 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 debug-
ging 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.
FIGURE 2-2:
VDD
EXAMPLE OF MCLR PIN
CONNECTIONS
R1
R2
MCLR
JP
PIC24FXXX
C1
Note 1:
2:
R1  10 k is recommended. A suggested
starting value is 10 k. Ensure that the MCLR
pin VIH and VIL specifications are met.
R2  470 will limit any current flowing into
MCLR from the external capacitor, C, in the
event of a MCLR pin breakdown, due to
Electrostatic Discharge (ESD) or Electrical
Overstress (EOS). Ensure that the MCLR pin
VIH and VIL specifications are met.
DS30010118B-page 30
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