W79E201 Datasheet, PDF (41/87 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E201
cleared by power on resets. This disables the Watchdog timer resets. A watchdog or external reset
does not affect the EWT bit.
10. INTERRUPTS
The W79E201 has a two priority level interrupt structure with 8 interrupt sources. Each of the interrupt
sources has an individual priority bit, flag, interrupt vector and enable bit. In addition, the interrupts can
be globally enabled or disabled.
Interrupt Sources
The External Interrupts INT0 and INT1 can be either edge triggered or level triggered, depending on
bits IT0 and IT1. The bits IE0 and IE1 in the TCON register are the flags which are checked to
generate the interrupt. In the edge triggered mode, the INTx inputs are sampled in every machine
cycle. If the sample is high in one cycle and low in the next, then a high to low transition is detected
and the interrupts request flag IEx in TCON is set. The flag bit requests the interrupt. Since the
external interrupts are sampled every machine cycle, they have to be held high or low for at least one
complete machine cycle. The IEx flag is automatically cleared when the service routine is called. If the
level triggered mode is selected, then the requesting source has to hold the pin low till the interrupt is
serviced. The IEx flag will not be cleared by the hardware on entering the service routine. If the
interrupt continues to be held low even after the service routine is completed, then the processor may
acknowledge another interrupt request from the same source.
The Timer 0 and 1 Interrupts are generated by the TF0 and TF1 flags. These flags are set by the
overflow in the Timer 0 and Timer 1. The TF0 and TF1 flags are automatically cleared by the
hardware when the timer interrupt is serviced. The Timer 2 interrupt is generated by a logical OR of
the TF2 and the EXF2 flags. These flags are set by overflow or capture/reload events in the timer 2
operation. The hardware does not clear these flags when a timer 2 interrupt is executed. Software has
to resolve the cause of the interrupt between TF2 and EXF2 and clear the appropriate flag.
The Watchdog timer can be used as a system monitor or a simple timer. In either case, when the
time-out count is reached, the Watchdog timer interrupt flag WDIF (WDCON.3) is set. If the interrupt is
enabled by the enable bit EIE.4, then an interrupt will occur.
All the bits that generate interrupts can be set or reset by hardware, and thereby software initiated
interrupts can be generated. Each of the individual interrupts can be enabled or disabled by setting or
clearing a bit in the IE SFR. IE also has a global enable/disable bit EA, which can be cleared to
disable all the interrupts.
Priority Level Structure
There are two priority levels for the interrupts, highest, high and low. The interrupt sources can be
individually set to either high or low levels. Naturally, a higher priority interrupt cannot be interrupted
by a lower priority interrupt. However there exists a pre-defined hierarchy amongst the interrupts
themselves. This hierarchy comes into play when the interrupt controller has to resolve simultaneous
requests having the same priority level. This hierarchy is defined as shown below; the interrupts are
numbered starting from the highest priority to the lowest.
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Publication Release Date: December 16, 2004
Revision A2