
www.national.com
32
Revision1.2
P
2.0
Power, Reset and Clocks
2.1
POWER
2.1.1
The PC8741x devices have three power planes (wells), as shown in the table below:
Power Planes
Table 4. Power Planes
For correct operation, either V
SB
or V
BAT
must be applied whenever V
DD
is applied.
2.1.2
The PC8741x devices have four power states:
Battery Fail
- the Main, Standby and Backup power planes are all powered off (V
DD
, V
SB
and V
BAT
are inactive).
Power Fail
- the Main and Standby power planes are powered off; the Backup power plane is on (V
DD
and V
SB
are
inactive; V
BAT
is active).
Power Off
-
the Main power plane is powered off; the Standby power plane is on; the Backup power plane is on (V
DD
is inactive; V
SB
is active; V
BAT
is irrelevant).
Power On
-
the Main and Standby power planes are powered on; the Backup power plane may is on (V
DD
and V
SB
are active; V
BAT
is irrelevant).
The following power state is illegal:
The Main power plane is powered on, the Standby power plane is off and the Backup power plane is on or off (i.e., V
DD
is active, V
SB
is inactive and V
BAT
is irrelevant).
The following table summarizes the power states described above.
Power States
Power Plane
Description
Power Pins
Ground Pins
Main
Powers the Legacy modules (Serial Ports, Parallel Port,
FDC, KBC), the LPC Interface, part of the Configuration
Control and some external signals
1
1. See the tables in Section 1.4 (pages 23-30), specifically the
Power Well
column.
2. V
PP
is an internal power signal derived from V
SB
or V
BAT
. V
PP
is taken from V
SB
if it is greater than
the minimum value defined in Section 11.1.5; otherwise it is taken from V
BAT
. For more
details on switching between them, refer to Section 8.2.9.
V
DD
V
SS
Standby
Powers the ACCESS.bus and X-Bus Interfaces, the GPIO
ports, the Clock Generator, part of the SWC, part of the
Configuration Control and some external related signals
1
V
SB
V
SS
Backup
Powers the RTC, the 32.768 KHz clock/crystal oscillator,
part of the SWC and some functions that must be
preserved at all times
1
V
PP2
V
SS
Table 5. Power States and Related Power Planes
Power State
Main (V
DD
)
Standby (V
SB
)
Backup (V
PP
)
V
BAT
Battery Fail
Power Fail
Power Off
Power On
Illegal
1
Off
Off
Off
Off
Off
Off
On
On
Off
On
On
On or Off
On
On
On
On or Off
1. Operation is not guaranteed and register data may be corrupted.
On
Off
On or Off
On or Off