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3-8 Intel® PXA255 Processor Developer’s Manual
Clocks and Power Manager
3.4.2.3 Completing a Watchdog Reset
Watchdog resets immediately revert to hardware resets when the nRESET pin is asserted.
Otherwise, the completion sequence for watchdog reset is:
1. The 3.6864 MHz oscillator and internal PLL clock generators wait for stabilization. The
32.768 kHz oscillator’s configuration and status are not affected by watchdog reset.
2. The nRESET_OUT pin is deasserted after t
DHW_OUT
. Refer to the Intel® PXA255 Processor
Electrical, Mechanical, and Thermal Specification.
3. The normal boot-up sequence begins. All processor units except the RTTR in the RTC and
parts of the Clocks and Power Manager return to their predefined reset conditions. Software
must examine the RCSR to determine the cause for the reboot.
3.4.3 GPIO Reset
A GPIO Reset is invoked when GP[1] is properly configured as a reset source and is asserted low
for greater than four 3.6864-MHz clock cycles.
In GPIO Reset all processor units except the RTC,
parts of the Clocks and Power Manager, and the Memory Controller return to their predefined,
known states.
3.4.3.1 Invoking GPIO Reset
To use the GPIO Reset function, set it up through the GPIO Controller. The GP[1] pin must be
configured as an input and set to its alternate GPIO Reset function in the GPIO Controller. The
GPIO Reset alternate function is level-sensitive and not edge-triggered. To ensure no spurious
resets are generated when the alternate GPIO Reset function is set, follow these steps:
1. GP[1] must be set up as an output with its data register set to a 1.
2. Externally drive the GP[1] pin to a high state.
3. Configure GP[1] as an input.
4. Configure GP[1] for its Alternate (Reset) Function.
The previous mode of operation does not affect a GPIO Reset. When performing a GPIO Reset,
nRESET_OUT is asserted.
If GP[1] is asserted for less than four 3.6864-MHz clock cycles, the
processor may remain in its previous mode or enter into a GPIO reset.
GPIO Reset does not function in Sleep Mode because all GPIO pins’ Alternate Function Inputs are
disabled. External wake-up sources must be routed through one of the enabled GPIO wake-up
sources (see Section 3.5.3 for details) during Sleep Mode. GP[1] may be enabled as a wake-up
source.
3.4.3.2 Behavior During GPIO Reset
During GPIO Reset, most, but not all, internal registers and processes are held at their defined reset
conditions. The exceptions are the RTC, the Clocks and Power Manager (unless otherwise noted),
and the Memory Controller. During GPIO Reset, the clocks unit continues to operate with its
previously programmed values, so the processor enters and exits GPIO Reset with the same clock
configurations. All pins except the oscillator and Memory Controller pins return to their reset
conditions and the nBATT_FAULT and nVDD_FAULT pins are ignored.