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EVENT PROCESSOR ARRAY (EPA)
10.8 SERVICING THE MULTIPLEXED EPA INTERRUPT WITH SOFTWARE
The multiplexed interrupts (those represented by EPAx) should be serviced with a standard inter-
rupt service routine rather than the PTS (Chapter 5, "Standard and PTS Interrupts"). The PTS can
take only a limited number of actions, while interrupt service routines can be tailored to the needs
of each interrupt.
The EPA_PEND (Figure 10-14) and EPA_PEND1 (Figure 10-15) registers contain the bits that
identify the interrupt source(s). Traditionally, software would sort these bits to determine which
interrupt service routine to execute. This sorting increases the overall interrupt response time by
a significant number of states. However, the EPA interrupt priority vector register (EPAIPV, Fig-
ure 10-16) contains a number that corresponds to the highest-priority active interrupt source (Ta-
ble 10-6).
For example, assume that an overrun occurs on capture/compare channel 9 and no other multi-
plexed interrupt is pending and unmasked. This sets the OVR9 pending bit in the EPA_PEND
register. If the corresponding mask bit is set in the EPA_MASK register, the EPAx interrupt pend-
ing bit is set. If enabled, the EPAx interrupt is generated. The encoder places the number for the
OVR9 interrupt (05H) into EPAIPV. Reading EPAIPV identifies capture/compare channel 9 as
the source, clears the OVR9 pending bit, and clears EPAIPV. When the device vectors to the EPAx
interrupt service routine, the EPAx pending bit is cleared. If other multiplexed interrupts have oc-
curred, the encoder loads the number that corresponds to the highest-priority, active, multiplexed
interrupt into EPAIPV. When the EPAIPV register contains 00H, there are no more pending in-
terrupts associated with the EPAx interrupt. Thus, it is recommended that the EPAIPV register be
read until it equals 00H to ensure that all pending, enabled interrupts are serviced.
10-29
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