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2-6 Intel® PXA255 Processor Developer’s Manual
System Architecture
Each interrupt goes through the Interrupt Controller Mask Register and then the Interrupt
Controller Level Register directs the interrupt into either the IRQ or FIQ. If an interrupt is taken,
the software may read the Interrupt Controller Pending Register to identify the source. After it
identifies the interrupt source, software is responsible for servicing the interrupt and clearing it in
the source unit before exiting the service routine.
Note: Clearing interrupts may take a delay. To allow the status bit to clear before returning from an
interrupt service routine (ISR), clear the interrupt early in the routine.
2.6 Reset
The processor can be reset in any of three ways: Hardware, Watchdog, and GPIO resets. Each is
described in more detail in Section 3.4, “Resets and Power Modes” on page 3-6.
• Hardware reset results from asserting the nRESET pin and forces all units into reset state.
• Watchdog reset results from a time-out in the OS Timer and may be used to recover from
runaway code. Watchdog reset is disabled by default and must be enabled by software.
• GPIO reset is a “soft reset” that is less destructive than Hardware and Watchdog resets.
Each type of reset affects the state of the processor pins. Table 2-4 shows each pin’s state after each
type of reset.
Leaving Sleep Mode causes a Sleep Mode reset. Unlike other resets, Sleep Mode resets do not
change the state of the pins.
The Reset Controller Status Register (RCSR) contains information on the type of reset, including
Sleep Mode resets.
Table 2-4. Effect of Each Type of Reset on Internal Register State (Sheet 1 of 2)
Unit Sleep Mode GPIO Reset Watchdog Reset Hard Reset
Core reset reset reset reset
Memory Controller reset preserved reset reset
LCD Controller reset reset reset reset
DMA Controller reset reset reset reset
Full Function UART reset reset reset reset
Bluetooth UART reset reset reset reset
Standard UART reset reset reset reset
Hardware UART reset reset reset reset
I
2
C reset reset reset reset
I
2
S reset reset reset reset
AC97 reset reset reset reset
USB reset reset reset reset
ICP reset reset reset reset
RTC preserved preserved reset (except RTTR) reset
OS Timer reset reset reset reset