MCP79400 datasheet(13/62 Pages) MICROCHIP

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DS20005009F-page 13

MCP79400/MCP79401/MCP79402

5.2

Oscillator Configuration

The MCP7940X can be operated in two different

oscillator configurations: using an external crystal or

using an external clock input.

5.2.1

EXTERNAL CRYSTAL

The crystal oscillator circuit on the MCP7940X is

designed to operate with a standard 32.768 kHz tuning

fork crystal and matching external load capacitors. By

using external load capacitors, the MCP7940X allows

for a wide selection of crystals. Suitable crystals have a

load capacitance (CL) of 6-9 pF. Crystals with a load

capacitance of 12.5 pF are not recommended.

Figure 5-2 shows the pin connections when using an

external crystal.

FIGURE 5-2:

CRYSTAL OPERATION

5.2.1.1

Choosing Load Capacitors

CL is the effective load capacitance as seen by the

crystal, and includes the physical load capacitors, pin

capacitance, and stray board capacitance. Equation 5-1

can be used to calculate CL.

CX1 and CX2 are the external load capacitors. They

must be chosen to match the selected crystal’s

specified load capacitance.

EQUATION 5-1:

LOAD CAPACITANCE

CALCULATION

5.2.1.2

Layout Considerations

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. 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 VSS.

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.

Layout suggestions are shown in Figure 5-3. In-line

packages may be handled with a single-sided layout

that completely encompasses the oscillator pins. With

fine-pitch packages, it is not always possible to

completely surround the pins and components. A

suitable solution is to tie the broken guard sections to a

mirrored ground layer. In all cases, the guard trace(s)

must be returned to ground.

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):

• AN1365, “Recommended Usage of Microchip

Serial RTCC Devices”

• AN1519, “Recommended Crystals for Microchip

Stand-Alone Real-Time Clock Calendar Devices”

Note 1:

The ST bit must be set to enable the

crystal oscillator circuit.

Always verify oscillator performance over

the voltage and temperature range that is

expected for the application.

Note:

If the load capacitance is not correctly

matched to the chosen crystal’s specified

value, the crystal may give a frequency

outside of the crystal manufacturer’s

specifications.

CX1

CX2

Quartz

To Internal

Logic

Crystal

MCP7940X



--------------------------

CSTRAY

Where:

Effective load capacitance

Capacitor value on X1

COSC

Capacitor value on X2

COSC

CSTRAY

PCB stray capacitance

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