PIC18F2221 datasheet(27/402 Pages) MICROCHIP

27 / 402 page

DS39689F-page 27

PIC18F2221/2321/4221/4321 FAMILY

2.4

ICSP Pins

The PGC and PGD pins are used for In-Circuit Serial

Programming (ICSP) and debugging purposes. It is

recommended to keep the trace length between the

ICSP connector and the ICSP pins on the device as

short as possible. If the ICSP connector is expected to

experience an ESD event, a series resistor is recom-

mended, with the value in the range of a few tens of

ohms, not to exceed 100Ω.

Pull-up resistors, series diodes and capacitors on the

PGC and PGD pins are not recommended as they will

interfere

with

the

programmer/debugger

com-

munications to the device. If such discrete components

are an application requirement, they should be removed

from the circuit during programming and debugging.

Alternatively, refer to the AC/DC characteristics and

timing requirements information in the respective device

Flash programming specification for information on

capacitive loading limits and pin input voltage high (VIH)

and input low (VIL) requirements.

For device emulation, ensure that the “Communication

Channel Select” (i.e., PGC/PGD pins) programmed

into the device matches the physical connections for

the ICSP to the MPLAB® ICD 2, MPLAB ICD 3 or REAL

ICE™ emulator.

For more information on the ICD 2, ICD 3 and REAL

ICE emulator connection requirements, refer to the

following documents that are available on the

Microchip web site.

• “MPLAB® ICD 2 In-Circuit Debugger User’s

Guide” (DS51331)

• “Using MPLAB® ICD 2” (poster) (DS51265)

• “MPLAB® ICD 2 Design Advisory” (DS51566)

• “Using MPLAB® ICD 3” (poster) (DS51765)

• “MPLAB® ICD 3 Design Advisory” (DS51764)

• “MPLAB® REAL ICE™ In-Circuit Emulator User’s

Guide” (DS51616)

• “Using MPLAB® REAL ICE™ In-Circuit Emulator”

(poster) (DS51749)

2.5

External Oscillator Pins

Many microcontrollers have options for at least two

oscillators: a high-frequency primary oscillator and a

low-frequency

secondary

oscillator

(refer

Section 3.0 “Oscillator Configurations” for details).

The oscillator circuit should be placed on the same

side of the board as the device. Place the oscillator

circuit close to the respective oscillator pins with no

more than 0.5 inch (12 mm) between the circuit

components and the pins. The load capacitors should

be placed next to the oscillator itself, on the same side

of the board.

Use a grounded copper pour around the oscillator

circuit to isolate it from surrounding circuits. The

grounded copper pour should be routed directly to the

MCU ground. Do not run any signal traces or power

traces inside the ground pour. Also, if using a

two-sided board, avoid any traces on the other side of

the board where the crystal is placed. A suggested

layout is shown in Figure 2-3.

For additional information and design guidance on

oscillator circuits, please refer to these Microchip

Application Notes, available at the corporate web site

(www.microchip.com):

• AN826, “Crystal Oscillator Basics and Crystal

Selection for rfPIC™ and PICmicro® Devices”

• AN849, “Basic PICmicro® Oscillator Design”

• AN943, “Practical PICmicro® Oscillator Analysis

and Design”

• AN949, “Making Your Oscillator Work”

FIGURE 2-3:

SUGGESTED PLACEMENT

OF THE OSCILLATOR

CIRCUIT

2.6

Unused I/Os

Unused I/O pins should be configured as outputs and

driven to a logic low state. Alternatively, connect a 1 kΩ

to 10 kΩ resistor to VSS on unused pins and drive the

output to logic low.

Main Oscillator

Guard Ring

Guard Trace

Secondary

Oscillator

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