TCD Counter Capture

The TCD counter is asynchronous to the peripheral clock, so it is not possible to read out the counter value directly. It is possible to capture the TCD counter value, synchronized to the I/O clock domain, in two ways:

The capture logic contains two separate capture blocks, CAPTUREA and CAPTUREB, that can capture and synchronize the TCD counter value to the I/O clock domain. CAPTUREA/B can be triggered by input event A/B or by software.

The capture values can be obtained by reading first the TCDn.CAPTUREAL/TCDn.CAPTUREBL registers and then the TCDn.CAPTUREAH/TCDn.CAPTUREBH registers.

Captures Triggered by Input Events

To enable the capture on an input event, write a ‘1’ to the ACTION bit in the respective Event Control (TCDn.EVCTRLA or TCDn.EVCTRLB) register when configuring an event input.

When a capture has occurred, the TRIGA/B flag is raised in the Interrupt Flags (TCDn.INTFLAGS) register. The corresponding TRIGA/B interrupt can be enabled by writing a ‘1’ to the respective Trigger Interrupt Enable (TRIGA or TRIGB) bit in the Interrupt Control (TCDn.INTCTRL) register. By polling TRIGA or TRIGB in TCDn.INTFLAGS, the user knows that a CAPTURE value is available and can read out the value by reading first the TCDn.CAPTUREAL or TCDn.CAPTUREBL register and then the TCDn.CAPTUREAH or TCDn.CAPTUREBH register.

PWM Capture

To perform a PWM capture, connect both event A and event B to the same asynchronous event channel that contains the PWM signal. To get information on the PWM signal, configure one event input to capture the rising edge of the signal. Configure the other event input to capture the falling edge of the signal.


▲ Event trigger

* Interrupt trigger

Capture Triggered by Software

The software can capture the TCD value by writing a ‘1’ to the respective Software Capture A/B Strobe (SCAPTUREx) bit in the Control E (TCDn.CTRLE) register. When this command is executed, and the Command Ready (CMDRDY) bit in the Status (TCDn.STATUS) register reads ‘1’ again, the CAPTUREA/B value is available. It can now be read by reading first the TCDn.CAPTUREAL or TCDn.CAPTUREBL register and then the TCDn.CAPTUREAH or the TCDn.CAPTUREBH register.

Using Capture Together with Input Modes

The capture functionality can be used together with the input modes. The same event will then both capture the counter value and trigger a change in the counter flow, depending on the input mode selected.

Reset One Ramp Mode by Input Event Capture

In One Ramp mode, the counter can be reset by an input event capture. To achieve this, use input event B and write 0x08 to the INPUTMODE bit field in the Input Control B (TCDn.INPUTCTRLB) register.