Notes on Printed Circuit Board Design

This section describes points that require care concerning noise from the point of view of the microcontroller and

presents means of preventing associated problems when designing a printed circuit board to use with these products in a

mass produced end product. The ideas presented in this section are effective design techniques for preventing and

avoiding problems (such as incorrect microcontroller operation and program failures) due to noise.

equal as possible (L1 = L1’, L2 = L2’). Furthermore, these distances should be as short as possible.

• Insert two capacitors, C1 and C2 in parallel, with C1 having a large capacitance and C2 having a small capacitance.

2. The OSC1 and OSC2 clock I/O pins

— If the ceramic oscillator option is selected (See figure 2-1.)

• Keep the lines between the clock I/O pins (input: OSC1, output:

OSC2) and the external components as short as possible (the

distance Losc in the figure).

• Make the length of the lines (Lvss + L1 and Lvss + L2) from

connected to the oscillator element as short as possible.

• Due to the capacitances of the wiring on the printed circuit

board, it may be necessary to modify the values of the oscillator

circuit constants (including the values of the capacitors C1 and

C2 and the limiting resistor Rd) from the values presented in

this catalog. We recommend consulting the manufacturer of the

oscillator element with regard to these circuit constants.

— If the 2-pin RC oscillator option is selected (Figure 2-2)

• Keep the lines between the clock I/O pins (input: OSC1, output:

OSC2) and the external components (the capacitor Cext and the

resistor Rext) as short as possible (the distance Losc in the figure).

• Make the length of the lines (Lvss + Lc) from the

functioning as the oscillator element as short as possible.

— If the external oscillator option is selected (Figure 2-3)

• Keep the line between the clock input pin (OSC1) and the external

oscillator circuit as short as possible (the distance Losc in the figure).

• Leave the clock output pin (OSC2) open.

used by the external oscillator as short as possible.

— Other points that apply to all oscillator circuits:

• Keep all lines that carry signals that change rapidly, signals that

have large amplitudes due to being connected to the medium-

voltage handling capacity ports, or signals that carry large

currents as far away from the oscillator circuit as possible. Also,

do not allow such signal lines to cross any clock-signal related

lines.

No. 6278-37/39

LC651154N, 651154F, 651154L, 651152N, 651152F, 651152L

Figure 2-2 Sample Oscillator Circuit 2

(2-pin RC oscillator)

Figure 2-1 Sample Oscillator Circuit 1

(Ceramic oscillator)

Figure 2-3 Sample Oscillator Circuit 3

(External oscillator)

External

oscillator