MPC7455 datasheet(52/64 Pages) FREESCALE

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MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1

Freescale Semiconductor

System Design Information

If address or data parity is not used by the system, and the respective parity checking is disabled through HID0, the

input receivers for those pins are disabled, and those pins do not require pull-up resistors and should be left

unconnected by the system. If all parity generation is disabled through HID0, then all parity checking should also

be disabled through HID0, and all parity pins may be left unconnected by the system.

The L3 interface does not normally require pull-up resistors.

9.7 JTAG Configuration Signals

Boundary-scan testing is enabled through the JTAG interface signals. The TRST signal is optional in the IEEE

1149.1 specification, but is provided on all processors that implement the PowerPC architecture. While it is possible

to force the TAP controller to the reset state using only the TCK and TMS signals, more reliable power-on reset

performance will be obtained if the TRST signal is asserted during power-on reset. Because the JTAG interface is

also used for accessing the common on-chip processor (COP) function, simply tying TRST to HRESET is not

practical.

The COP function of these processors allows a remote computer system (typically, a PC with dedicated hardware

and debugging software) to access and control the internal operations of the processor. The COP interface connects

primarily through the JTAG port of the processor, with some additional status monitoring signals. The COP port

requires the ability to independently assert HRESET or TRST in order to fully control the processor. If the target

system has independent reset sources, such as voltage monitors, watchdog timers, power supply failures, or

push-button switches, then the COP reset signals must be merged into these signals with logic.

The arrangement shown in Figure 26 allows the COP port to independently assert HRESET or TRST, while ensuring

that the target can drive HRESET as well. If the JTAG interface and COP header will not be used, TRST should be

tied to HRESET through a 0-

Ω isolation resistor so that it is asserted when the system reset signal (HRESET) is

asserted, ensuring that the JTAG scan chain is initialized during power-on. While Freescale recommends that the

COP header be designed into the system as shown in Figure 26, if this is not possible, the isolation resistor will allow

future access to TRST in the case where a JTAG interface may need to be wired onto the system in debug situations.

The COP header shown in Figure 26 adds many benefits—breakpoints, watchpoints, register and memory

examination/modification, and other standard debugger features are possible through this interface—and can be as

inexpensive as an unpopulated footprint for a header to be added when needed.

The COP interface has a standard header for connection to the target system, based on the 0.025" square-post, 0.100"

centered header assembly (often called a Berg header). The connector typically has pin 14 removed as a connector

key.

There is no standardized way to number the COP header shown in Figure 26; 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 26 is common to all known

emulators.

The QACK signal shown in Figure 26 is usually connected to the PCI bridge chip in a system and is an input to the

MPC7455 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 MPC7455 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 de-asserted

when it is not being driven by the tool. Note that the pull-up and pull-down resistors on the QACK signal are

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