MPC755 datasheet(44/56 Pages) NXP

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MPC755 RISC Microprocessor Hardware Specifications, Rev. 8

Freescale Semiconductor

System Design Information

There is no standardized way to number the COP header shown in Figure 24; consequently, many different

pin numbers have been observed from emulator vendors. Some are numbered top-to-bottom then

left-to-right, while others use left-to-right then top-to-bottom, while still others number the pins counter

clockwise from pin 1 (as with an IC). Regardless of the numbering, the signal placement recommended in

Figure 25 is common to all known emulators.

The QACK signal shown in Figure 24 is usually connected to the PCI bridge chip in a system and is an

input to the MPC755 informing it that it can go into the quiescent state. Under normal operation this occurs

during a low-power mode selection. In order for COP to work, the MPC755 must see this signal asserted

(pulled down). While shown on the COP header, not all emulator products drive this signal. If the product

does not, a pull-down resistor can be populated to assert this signal. Additionally, some emulator products

implement open-drain type outputs and can only drive QACK asserted; for these tools, a pull-up resistor

can be implemented to ensure this signal is deasserted when it is not being driven by the tool. Note that the

pull-up and pull-down resistors on the QACK signal are mutually exclusive and it is never necessary to

populate both in a system. To preserve correct power-down operation, QACK should be merged via logic

so that it also can be driven by the PCI bridge.

8.8

Thermal Management Information

This section provides thermal management information for air-cooled applications. Proper thermal control

design is primarily dependent on the system-level design—the heat sink, airflow, and thermal interface

material. To reduce the die-junction temperature, heat sinks may be attached to the package by several

methods—adhesive, spring clip to holes in the printed-circuit board or package, and mounting clip and

screw assembly; see Figure 25. This spring force should not exceed 5.5 pounds (2.5 kg) of force.

Figure 25 describes the package exploded cross-sectional view with several heat sink options.

Figure 25. Package Exploded Cross-Sectional View with Several Heat Sink Options

Adhesive or

Thermal Interface Material

Heat Sink

CBGA Package

Heat Sink

Clip

Printed-Circuit Board

Option

MPC755 Datasheet(PDF) 44 Page - NXP Semiconductors