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DS30010118D-page 30

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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:

EXAMPLE OF MCLR PIN

CONNECTIONS

Note 1: R1

 10 k is recommended. A suggested

starting value is 10 k

. Ensure that the MCLR

pin VIH and VIL specifications are met.

2: 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.

VDD

MCLR

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