w77e58 Winbond Electronics Corp America, w77e58 Datasheet - Page 52

• Manufacturer Part Number: w77e58
• Description: 8 Bit Microcontroller
• Manufacturer: Winbond Electronics Corp America

The interrupt flags are sampled every machine cycle. In the same machine cycle, the sampled
interrupts are polled and their priority is resolved. If certain conditions are met then the hardware will
execute an internally generated LCALL instruction which will vector the process to the appropriate
interrupt vector address. The conditions for generating the LCALL are
1. An interrupt of equal or higher priority is not currently being serviced.
2. The current polling cycle is the last machine cycle of the instruction currently being executed.
3. The current instruction does not involve a write to IP, IE, EIP or EIE registers and is not a RETI.
If any of these conditions are not met, then the LCALL will not be generated. The polling cycle is
repeated every machine cycle, with the interrupts sampled in the same machine cycle. If an interrupt
flag is active in one cycle but not responded to, and is not active when the above conditions are met,
the denied interrupt will not be serviced. This means that active interrupts are not remembered; every
polling cycle is new.
The processor responds to a valid interrupt by executing an LCALL instruction to the appropriate
service routine. This may or may not clear the flag which caused the interrupt. In case of Timer
interrupts, the TF0 or TF1 flags are cleared by hardware whenever the processor vectors to the
appropriate timer service routine. In case of external interrupt, INT0 and INT1, the flags are cleared
only if they are edge triggered. In case of Serial interrupts, the flags are not cleared by hardware. In
the case of Timer 2 interrupt, the flags are not cleared by hardware. The Watchdog timer interrupt
flag WDIF has to be cleared by software. The hardware LCALL behaves exactly like the software
LCALL instruction. This instruction saves the Program Counter contents onto the Stack, but does not
save the Program Status Word PSW. The PC is reloaded with the vector address of that interrupt
which caused the LCALL. These vector address for the different sources are as follows
Table 8. Vector locations for interrupt sources
The vector table is not evenly spaced; this is to accommodate future expansions to the device family.
Source
Timer 0 Overflow
Timer 1 Overflow
Timer 2 Interrupt
External Interrupt 2
External Interrupt 4
Watchdog Timer
Vector Address
000Bh
001Bh
002Bh
0043h
0053h
0063h
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Source
External Interrupt 0
External Interrupt 1
Serial Port
Serial Port 1
External Interrupt 3
External Interrupt 5
Preliminary W77E58
Vector Address
0003h
0013h
0023h
003Bh
004Bh
005Bh