MSC8122 datasheet(84/88 Pages) FREESCALE

84 / 88 page

MSC8122 Technical Data, Rev. 13

4-2

Freescale Semiconductor

Design Considerations

•

Decouple the supply using low-ESR capacitors mounted as close as possible to the socket. Figure 4-1 shows

three capacitors in parallel to reduce the resistance. Three capacitors is a recommended minimum number. If

possible, mount at least one of the capacitors directly below the MSC8122 device.

Each VCC and VDD pin on the MSC8122 device should have a low-impedance path to the board power supply.

Similarly, each GND pin should have a low-impedance path to the ground plane. The power supply pins drive

distinct groups of logic on the chip. The VCC power supply should have at least four 0.1 µF by-pass capacitors to

ground located as closely as possible to the four sides of the package. The capacitor leads and associated printed

circuit traces connecting to chip VCC, VDD, and GND should be kept to less than half an inch per capacitor lead. A

four-layer board is recommended, employing two inner layers as VCC and GND planes.

All output pins on the MSC8122 have fast rise and fall times. PCB trace interconnection length should be

minimized to minimize undershoot and reflections caused by these fast output switching times. This

recommendation particularly applies to the address and data buses. Maximum PCB trace lengths of six inches are

recommended. For the DSI control signals in synchronous mode, ensure that the layout supports the DSI AC

timing requirements and minimizes any signal crosstalk. Capacitance calculations should consider all device loads

as well as parasitic capacitances due to the PCB traces. Attention to proper PCB layout and bypassing becomes

especially critical in systems with higher capacitive loads because these loads create higher transient currents in the

VCC, VDD, and GND circuits. Pull up all unused inputs or signals that will be inputs during reset.

Special care should be taken to minimize the noise levels on the PLL supply pins. There is one pair of PLL supply

pins: VCCSYN-GNDSYN. To ensure internal clock stability, filter the power to the VCCSYN input with a circuit similar to

the one in Figure 4-2. For optimal noise filtering, place the circuit as close as possible to VCCSYN. The 0.01-µF

capacitor should be closest to VCCSYN, followed by the 10-µF capacitor, the 10-nH inductor, and finally the 10-

resistor to VDD. These traces should be kept short and direct. Provide an extremely low impedance path to the

ground plane for GNDSYN. Bypass GNDSYN to VCCSYN by a 0.01-µF capacitor located as close as possible to the chip

package. For best results, place this capacitor on the backside of the PCB aligned with the depopulated void on the

MSC8122 located in the square defined by positions, L11, L12, L13, M11, M12, M13, N11, N12, and N13.

Figure 4-1.

Core Power Supply Decoupling

Figure 4-2.

VCCSYN Bypass

Power supply

Voltage Regulator

High frequency capacitors

(very low ESR and ESL)

Bulk/Tantalum capacitors

with low ESR and ESL

MSC8122

Maximum IR drop

of 15 mV at 1 A

Note: Use at least three capacitors.

Lmax = 2 cm

One 0.01 µF capacitor

for every 3 core supply

(Imin = 3 A)

pads.

1.2 V

Each capacitor must be at least 150

μF.

VDD

0.01 µF

10 µF

VCCSYN

10nH

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