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Clocking and Resets

9.4Reset modes

The reset signals present in the processor design enable you to reset different parts of the design independently. Table 9-1 lists the reset signals, and the combinations and possible applications that you can use them in.

 

 

 

 

Table 9-1 Reset modes

 

 

 

 

 

Reset mode

nRESETIN

DBGnTRST

nPORESETIN

Application

 

 

 

 

 

Power-on reset

0

x

0

Reset at power up, full system reset.

 

 

 

 

Hard reset or cold reset.

 

 

 

 

 

Processor reset

0

x

1

Reset of processor core only, watchdog

 

 

 

 

reset.

 

 

 

 

Soft reset or warm reset.

 

 

 

 

 

DBGTAP reset

1

0

1

Reset of DBGTAP logic.

 

 

 

 

 

Normal

1

x

1

No reset. Normal run mode.

 

 

 

 

 

9.4.1Power-on reset

You must apply power-on or cold reset to the processor when power is first applied to the system. In the case of power-on reset, the leading, falling, edge of the reset signals, nRESETIN and nPORESETIN, does not have to be synchronous to CLKIN. Because the nRESETIN and nPORESETIN signals are synchronized within the processor, you do not have to synchronize

these signals. Figure 9-6 shows the application of power-on reset.

CLKIN

nRESETIN

nPORESETIN

Figure 9-6 Power-on reset

It is recommended that you assert the reset signals for at least three CLKIN cycles to ensure correct reset behavior. Adopting a three-cycle reset eases the integration of other ARM parts into the system, for example, ARM9TDMI-based designs.

It is not necessary to assert DBGnTRST on power-up.

9.4.2CP14 debug logic

Because the nPORESETIN signal is synchronized within the processor, you do not have to synchronize this signal.

9.4.3Processor reset

A processor or warm reset initializes the majority of the ARM1176JZ-S processor, excluding the ARM1176JZ-S DBGTAP controller and the EmbeddedICE-RT logic. Processor reset is typically used for resetting a system that has been operating for some time, for example, watchdog reset.

Because the nRESETIN signal is synchronized within the processor, you do not have to synchronize this signal.

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Clocking and Resets

9.4.4DBGTAP reset

DBGTAP reset initializes the state of the processor DBGTAP controller. DBGTAP reset is typically used by the RealView ICE module for hot connection of a debugger to a system.

DBGTAP reset enables initialization of the DBGTAP controller without affecting the normal operation of the processor.

Because the DBGnTRST signal is synchronized within the processor, you do not have to synchronize this signal.

9.4.5Normal operation

During normal operation, neither processor reset nor power-on reset is asserted. If the DBGTAP port is not being used, the value of DBGnTRST does not matter.

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