Renesas 7700 FAMILY User Manual page 67

INTERRUPTS
4.5 Interrupt priority level detection circuit
The following explains the operation of the interrupt priority detection circuit using Figure 4.5.2.
The interrupt priority level of a requested interrupt (Y in Figure 4.5.2) is compared with the resultant priority
level sent from the preceding comparator (X in Figure 4.5.2); whichever interrupt of the higher priority level
is sent to the next comparator (Z in Figure 4.5.2). (Initial comparison value is "0.") For interrupts for which
no interrupt request occurs, the priority level sent from the preceding comparator is forwarded to the next
comparator. When the two priority levels are found the same by comparison, the priority level sent from the
preceding comparator is forwarded to the next comparator. Accordingly, when the same priority level is set
by software, the interrupt requests are subject to the following relation about priority:
A-D conversion > UART1 transmit > UART1 receive > UART0 transmit > UART0 receive > Timer B2
> Timer B1 > Timer B0 > Timer A4 > Timer A3 > Timer A2 > Timer A1 > Timer A0 > INT
Among the multiple interrupt requests sampled at the same time, one that has the highest priority level is
detectedd by the above comparison.
Then this highest interrupt priority level is compared with the processor interrupt priority level (IPL). When
this interrupt priority level is higher than the processor interrupt priority level (IPL) and the interrupt disable
flag (I) is "0," the interrupt request is accepted. A interrupt request which is not accepted here is retained
until it is accepted or its interrupt request bit is cleared to "0" by software.
The interrupt priority is detected when the CPU fetches an op code, which is called the CPU's op-code fetch
cycle. However, when an op-code fetch cycle is generated during detection of an interrupt priority, new
detection of that does not start. (Refer to Figure 4.6.1.) Since the state of the interrupt request bit and
interrupt priority levels are latched during detection of interrupt priority, even if the bit state and priority
levels change, the detection is performed on the previous state before it has changed.
Interrupt source Y
Fig. 4.5.2 Interrupt priority level detection model
4–12
X
Time
Y
Comparator
(Priority level
comparison)
Z
7751 Group User's Manual
X : Resultant priority level sent from the preceding
comparator (Highest priority at this point)
Y : Priority level of interrupt source Y
Z : Highest priority at this point
When X Y then Z = X
When X < Y then Z = Y
____ ____ ____
> INT > INT
2 1 0