Intermec 073290-001 User Manual

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 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: 

OK 

Not important 

Low-Z 

Controlled by software 

Critical 

Not important 

High-Z 

Off 

 

 

 

 

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.