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8XC196MC, MD, MH USER’S MANUAL
11-12
An input capture event does not set the interrupt pending bit until the captured time value actually
moves from the capture buffer into the EPAx_TIME register. If the buffer contains data and the
PTS is used to service the interrupts, then two PTS interrupts occur almost back-to-back (that is,
with one instruction executed between the interrupts).
11.4.1.1 EPA Overruns
Overruns occur when an EPA input transitions at a rate that cannot be handled by the EPA inter-
rupt service routine. If no overrun handling strategy is in place, and if the following three condi-
tions exist, a situation may occur where both the capture buffer and the EPAx_TIME register
contain data, and no EPA interrupt is generated.
• an input signal with a frequency high enough to cause overruns is present on an enabled
EPA pin, and
• the overwrite bit is set (EPAx_CON.0 = 1; old data is overwritten on overrun), and
• the EPAx_TIME register is read at the exact instant that the EPA recognizes the captured
edge as valid.
The input frequency at which this occurs depends on the length of the interrupt service routine as
well as other factors. Unless the interrupt service routine includes a check for overruns, this situ-
ation will remain the same until the device is reset or the EPAx_TIME register is read. The act of
reading EPAx_TIME allows the buffered time value to be moved into EPAx_TIME. This clears
the buffer and allows another event to be captured. Remember that the act of the transferring the
buffer contents to the EPAx_TIME register is what actually sets the EPAx interrupt pending bit
and generates the interrupt.
Table 11-5. Action Taken When a Valid Edge Occurs
Overwrite Bit
(EPA
x
_CON.0)
Status of
Capture Buffer
& EPA
x
_TIME
Action Taken When a Valid Edge Occurs
0 empty Edge is captured and event time is loaded into the capture buffer and
EPA
x
_TIME register.
0 full New data is ignored — no capture, EPA interrupt, or transfer occurs.
1 empty Edge is captured and event time is loaded into the capture buffer and
EPA
x
_TIME register.
1 full Old data is overwritten in the capture buffer.