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COP87L88GG Datasheet(PDF) 16 Page - National Semiconductor (TI) |
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COP87L88GG Datasheet(HTML) 16 Page - National Semiconductor (TI) |
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16 / 42 page  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 readwrite 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 WakeupInterrupt 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 WKEN WKPND and WKEDG are all readwrite 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 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 (See HALT MODE for clock option wakeup informa- tion) http www nationalcom 16 |
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