C8051F850-C-GM Datasheet, PDF (168/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

20.3.2. Edge-Triggered Capture Mode
In this mode, a valid transition on the CEXn pin causes the PCA to capture the value of the PCA counter/
timer and load it into the corresponding module's 16-bit capture/compare register (PCA0CPLn and
PCA0CPHn). The CAPPn and CAPNn bits in the PCA0CPMn register are used to select the type of
transition that triggers the capture: low-to-high transition (positive edge), high-to-low transition (negative
edge), or either transition (positive or negative edge). When a capture occurs, the Capture/Compare Flag
(CCFn) in PCA0CN is set to logic 1. An interrupt request is generated if the CCFn interrupt for that module
is enabled. The CCFn bit is not automatically cleared by hardware when the CPU vectors to the interrupt
service routine, and must be cleared by software. If both CAPPn and CAPNn bits are set to logic 1, then
the state of the Port pin associated with CEXn can be read directly to determine whether a rising-edge or
falling-edge caused the capture.
CCFn (Interrupt Flag)
CEXn
CAPPn
PCA0CPLn PCA0CPHn
Capture
CAPNn
PCA Clock
PCA0L
PCA0H
Figure 20.2. PCA Capture Mode Diagram
Note: The CEXn input signal must remain high or low for at least 2 system clock cycles to be recognized by the
hardware.
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Rev. 1.0