COP87L88CL Datasheet, PDF (14/34 Pages) National Semiconductor (TI) – 8-Bit One-Time Programmable OTP Microcontroller

Multi-Input Wakeup (Continued)
The Multi-Input Wakeup 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 Reg WKEN The Reg
WKEN is an 8-bit read write register which contains a con-
trol bit for every L port bit Setting a particular WKEN bit
enables a Wakeup 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 Reg 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
pseudo Wakeup 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 re-enabled
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
SBIT 5 WKEN
If the L port bits have been used as outputs and then
changed to inputs with Multi-Input Wakeup Interrupt a safe-
ty procedure should also be followed to avoid inherited
pseudo wakeup conditions After the selected L port bits
have been changed from output to input but before the as-
sociated WKEN bits are enabled the associated edge se-
lect 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 Wakeup 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 wakeup conditions
the device will not enter the HALT mode if any Wakeup bit is
both enabled and pending Consequently the user has the
responsibility of clearing the pending flags before attempt-
ing to enter the HALT mode
The WKEN WKPND and WKEDG are all read write regis-
ters 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 inter-
rupt enable for Port L interrupts Setting the LPEN flag will
enable interrupts 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 IDLE 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
The Wakeup signal will not start the chip running immediate-
ly since crystal oscillators or ceramic resonators have a fi-
nite start up time The IDLE Timer (T0) generates a fixed
delay to ensure that the oscillator has indeed stabilized be-
fore allowing the execution of instructions In this case
upon detecting a valid Wakeup signal only the oscillator
circuitry and the IDLE Timer T0 are enabled The IDLE Tim-
er is loaded with a value of 256 and is clocked from the t_c
instruction cycle clock The t_c clock is derived by dividing
down the oscillator clock by a factor of 10 A Schmitt trigger
following the CKI on-chip inverter ensures that the IDLE tim-
er is clocked only when the oscillator has a sufficiently large
amplitude to meet the Schmitt trigger specifications This
Schmitt trigger is not part of the oscillator closed loop The
startup timeout from the IDLE timer enables the clock sig-
nals to be routed to the rest of the chip
If the RC clock option is used the fixed delay is under soft-
ware control A control flag CLKDLY in the G7 configura-
tion bit allows the clock start up delay to be optionally insert-
ed Setting CLKDLY flag high will cause clock start up delay
to be inserted and resetting it will exclude the clock start up
delay The CLKDLY flag is cleared during reset so the clock
start up delay is not present following reset with the RC
clock options
Interrupts
The device supports a vectored interrupt scheme It sup-
ports a total of ten interrupt sources The following table
lists all the possible interrupt sources their arbitration rank-
ing and the memory locations reserved for the interrupt vec-
tor for each source
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