52.6.1 Principle of Operation

For better understanding and to ease reading, the following description uses an example with a CMOS digital image sensor.

The CMOS digital image sensor provides a sensor clock, an 10-bit data synchronous with the sensor clock and two data enables which are also synchronous with the sensor clock.

The PCC must be configured first, and is enabled by writing a '1' to the Parallel Capture Enable bit in the Mode Register (MR.PCEN).

Once enabled, the PCC samples the data at rising edge of the sensor clock, and resynchronizes it with the PCC clock domain.

The input data bus size can be programmed using the Input Data Size bit field (MR.ISIZE).

A re-initialization of the internal mechanism of the PCC can be automatically done by setting the CID register when a falling edge of the DEN1 or DEN2 is detected. This feature allows glitch filtering and prevents image de-synchronization.

The number of the data which can be read in the Reception Holding Register (RHR) can be programmed by writing the Data Size bit field (MR.DSIZE). The PCC samples one or several sensor data, according to the DSIZE value.

If the MR.SCALE bit is written to '1' and MR.ISIZE ≠ 0, the sampled data is automatically up-scaled to 16 bits. When the right number of data has be sampled, data are stored in the RHR, and the Data Ready flag in the Interrupt Status Register (ISR.DRDY) is set to '1'.

The PCC can be associated with a reception channel of the DMA Controller (DMAC). This performs reception transfer from the PCC to a memory buffer without any intervention from the CPU.

The PCC can be configured to either comply with the sensor data enable signals, or not. If the Always Sampling bit in the Mode Register (MR.ALWYS) is written to '0', the PCC samples the sensor data at the rising edge of the sensor clock only if both data enable signals are active (at '1'). If ALWYS is written to '1', the PCC samples the sensor data at the rising edge of the sensor clock, independent of the data enable signals.

The PCC can be configured to sample the sensor data only every other time. This is particularly useful when only the luminance Y from a YUV422 data stream of a CMOS digital image sensor is to be sampled. If the Half Sampling bit in the Mode Register (MR.HALFS) is written to '0', the PCC samples the sensor data as configured above. If MR.HALFS=1, the PCC samples the sensor data as configured above (i.e. respecting the MR.ALWYS setting), but only one time out of two.

The PCC can either sample the even or odd sensor data, depending on the First Sample bit (MR.FRSTS). If sensor data are numbered with an index from zero to n in the order they are received and FRSTS=0, only data with an even index are sampled. For FRSTS=1, only data with an odd index are sampled.

If data are ready in the Reception Holding Register (RHR) but it is not read before new data is stored in RHR, an overrun error occurs: The previous data is lost and the Overrun Error flag in the Interrupt Status Register (ISR.OVRE) is set. This flag is automatically cleared when ISR is read (reset after read).

The flags DRDY and OVRE can be a source of the PCC interrupt.