Intermec CK30 Service Manual page 68

Chapter 4 — Theory of Operation
58
Device Power Control
Peripheral devices both internal and external to the PXA255 are enabled
and disabled under software control. External devices also have hardware
shutdown features to cut their power quickly in a critically-low battery
situation (low-battery hardware detect has tripped). In normal suspends,
power to these devices is turned off under software control.
These devices fall in 2 categories, depending on whether their functions
can tolerate sudden removal of power without time for software to clean
up. See Critical Shutdown Type column in the Device Power States table
on the previous page. Both types include devices that may optionally be
left on during Suspend. Regardless of shutdown type, those devices must
be dumped immediately if the battery goes critical (or is removed) while
the system is suspended.
Type 1
Type 1 devices are those whose power can be cut immediately on a
critically-low battery event. These are generally non-essential loads (for
example, the display backlight) that can be dumped right away to reduce
the load on a critically-low main battery, or on the supercap if the main
battery is being removed.
These loads are controlled through external hardware control registers
(HCRs) U11 and U16. In a critical battery situation, signal
BATT_FAULT_IRQ* is driven low by comparator U36. Through U52
and U53, this asserts signal HCR_DISABLE, which forces U11 and U16
outputs to High-Z. Pull-ups and pull-downs on these outputs then return
the devices to their default (off) state:
Battery State System State
OK Not important
Critical Not important
Type 2
Type 2 devices are those whose power cannot be cut immediately on
critically-low battery event.These are loads that could cause data
corruption if their power is removed before software can shut down cleanly
(for example, the SD slot), but that must be powered down right away on
a critically-low main event once the system is suspended. These are kept
powered as long as the PXA255 is awake so that their device drivers can
complete any critical operations and then shut down cleanly.
These loads are controlled through PXA255 GPIO registers so that they
will not automatically shut down the instant the battery goes critical. In a
critical battery situation, signal BATT_FAULT_IRQ* is driven low by
comparator U36 replaced with signal RESET_OUT* when the PXA255
suspends.
HCR Outputs Device Power
Low-Z Controlled by software
High-Z Off
CK30 Handheld Computer Service Manual