PIC18F86J15-I/PT【PDF 24/44 页】IC PIC MCU FLASH 48KX16 80TQFP

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PIC18F87J10 FAMILY

DS39663F-page 28

2009 Microchip Technology Inc.

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

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.

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.

2.3

Master Clear (MCLR) Pin

The MCLR pin provides two specific device

functions: device Reset, and device programming

and debugging. If programming and debugging are

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

beneficial.

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

debugging operations by using a jumper (Figure 2-2).

The

jumper

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 kΩ is recommended. A suggested

starting value is 10 k

Ω. Ensure that the

MCLR pin VIH and VIL specifications are met.

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.

VDD

MCLR

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