Compatible Systems RISC 2800i Instrukcja Użytkownika Pobierz pdf (Strona 37)

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5

Freescale Semiconductor 37

System Design Information

It is imperative to note that the processor’s minimum and maximum SYSCLK, core, and VCO frequencies

must not be exceeded regardless of the type of clock source. Therefore, systems in which the processor is

operated at its maximum rated core or bus frequency should avoid violating the stated limits by using

down-spreading only.

9.2 PLL Power Supply Filtering

The AV

power signal is provided on the MPC7447A to provide power to the clock generation PLL. To

ensure stability of the internal clock, the power supplied to the AV

input signal should be filtered of any

noise in the 500-KHz to 10-MHz resonant frequency range of the PLL. A circuit similar to the one shown

in Figure 17 using surface-mount capacitors with minimum effective series inductance (ESL) is

recommended.

The circuit should be placed as close as possible to the AV

pin to minimize noise coupled from nearby

circuits. It is often possible to route directly from the capacitors to the AV

pin, which is on the periphery

of the 360 HCTE footprint.

Figure 17. PLL Power Supply Filter Circuit

9.3 Decoupling Recommendations

Due to the MPC7447A dynamic power management feature, large address and data buses, and high

operating frequencies, the MPC7447A can generate transient power surges and high frequency noise in its

power supply, especially while driving large capacitive loads. This noise must be prevented from reaching

other components in the MPC7447A system, and the MPC7447A itself requires a clean, tightly regulated

source of power. Therefore, it is recommended that the system designer use sufficient decoupling

capacitors, typically one capacitor for every 1–2 V

pins, and a similar or lesser number for the OV

pins, placed as close as possible to the power pins of the MPC7447A. It is also recommended that these

decoupling capacitors receive their power from separate V

, OV

, and GND power planes in the PCB,

utilizing short traces to minimize inductance.

Table 14. Spread Specturm Clock Source Recommendations

At recommended operating conditions. See Ta b le 4 .

Parameter Min Max Unit Notes

Frequency modulation — 50 kHz 1

Frequency spread — 1.0 % 1, 2

Notes:

1. Guaranteed by design.

2. SYSCLK frequencies resulting from frequency spreading, and the resulting core and VCO

frequencies, must meet the minimum and maximum specifications given in Table 8.

10 Ω

2.2 µF 2.2 µF

GND

Low ESL Surface Mount Capacitors

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 55 56

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