64.6.7 Pad Calibration

The need for output impedance calibration arises with higher data rates. As the data rate increases, some transmission line effects may occur and lead to the generation of undershoots and overshoots, hence degrading the signal quality.

To avoid these transmission problems, a pad calibration cell is used to adjust the output impedance to the driven I/Os.

The pad calibration sequence can be started manually by writing a 1 to QSPI_CR.STPCAL. Then QSPI_SR.CALBSY indicates the state of the I/O calibration. It is advised to run manual pad calibration while the QSPI is disabled (QSPI_SR.QSPIENS=0) to avoid conflicts with the automatic refresh sequence.

The pad calibration sequence can also be performed automatically through a refresh. See Refresh Sequence.

The pad calibration cell requires a start-up time defined by the CALCNT field in the Pad Calibration Configuration register (QSPI_PCALCFG). The default value of this field is adequate for start-up. A different value may be configured in CALCNT.

If QSPI_PCALCFG.AAON is set to 1, the analog circuitry is not shut down at the end of the calibration and the start-up time is only required for the first calibration sequence, thus increasing performance. If QSPI_PCALCFG.AAON is set to 0, the analog circuitry is shut down at the end of the calibration sequence and power consumption is reduced. In this case, the start-up time is performed each time a calibration sequence is started and performance is reduced.

Automatic I/O calibration during the refresh sequence is disabled by writing QSPI_PCALCFG.DAPCAL to 1. For more details, see Refresh Sequence.