COP888GW Datasheet, PDF (21/44 Pages) National Semiconductor (TI) – 8-Bit Microcontroller with Pulse Train Generators

Multi-Input Wakeup (Continued)
The Multi-Input Wake Up feature utilizes the L Port The
user selects which particular L port bit (or combination of L
Port bits) will cause the device to exit the HALT or IDLE
modes The selection is done through the register WKEN
The register WKEN is an 8-bit read write register which
contains a control bit for every L port bit Setting a particular
WKEN bit enables a Wake Up from the associated L port
pin
The user can select whether the trigger condition on the
selected L Port pin is going to be either a positive edge (low
to high transition) or a negative edge (high to low transition)
This selection is made via the register WKEDG which is an
8-bit control register with a bit assigned to each L Port pin
Setting the control bit will select the trigger condition to be a
negative edge on that particular L Port pin Resetting the bit
selects the trigger condition to be a positive edge Changing
an edge select entails several steps in order to avoid a
Wake Up condition as a result of the edge change First the
associated WKEN bit should be reset followed by the edge
select change in WKEDG Next the associated WKPND bit
should be cleared followed by the associated WKEN bit
being reenabled
An example may serve to clarify this procedure Suppose
we wish to change the edge select from positive (low going
high) to negative (high going low) for L Port bit 5 where bit 5
has previously been enabled for an input interrupt The pro-
gram would be as follows
RBIT 5 WKEN
SBIT 5 WKEDG
RBIT 5 WKPND
SB1T 5 WKEN
If the L port bits have been used as outputs and then
changed to inputs with Multi-Input Wake Up lnterrupt a
safety procedure should also be followed to avoid wakeup
conditions After the selected L port bits have been
changed from output to input but before the associated
WKEN bits are enabled the associated edge select bits in
WKEDG should be set or reset for the desired edge selects
followed by the associated WKPND bits being cleared
This same procedure should be used following reset since
the L port inputs are left floating as a result of reset
The occurrence of the selected trigger condition for Multi-In-
put Wake Up is latched into a pending register called
WKPND The respective bits of the WKPND register will be
set on the occurrence of the selected trigger edge on the
corresponding Port L pin The user has the responsibility of
clearing these pending flags Since WKPND is a pending
register for the occurrence of selected wake up conditions
the device will not enter the HALT mode if any Wake Up bit
is both enabled and pending Consequently the user must
clear the pending flags before attempting to enter the HALT
mode
WKEN WKPND and WKEDG are all read write registers
and are cleared at reset
PORT L INTERRUPTS
Port L provides the user with an additional eight fully select-
able edge sensitive interrupts which are all vectored into
the same service subroutine
The interrupt from Port L shares logic with the wake up cir-
cuitry The register WKEN allows interrupts from Port L to
be individually enabled or disabled The register WKEDG
specifies the trigger condition to be either a positive or a
negative edge Finally the register WKPND latches in the
pending trigger conditions
The GIE (Global Interrupt Enable) bit enables the interrupt
function
A control flag LPEN functions as a global interrupt enable
for Port L interrupts Setting the LPEN flag will enable inter-
rupts and vice versa A separate global pending flag is not
needed since the register WKPND is adequate
Since Port L is also used for waking the device out of the
HALT or lDLE modes the user can elect to exit the HALT or
IDLE modes either with or without the interrupt enabled If
he elects to disable the interrupt then the device will restart
execution from the instruction immediately following the in-
struction that placed the microcontroller in the HALT or
IDLE modes In the other case the device will first execute
the interrupt service routine and then revert to normal oper-
ation (See HALT MODE for clock option wake up informa-
tion )
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