8.2.4.2 SDMMC I/O Calibration

The embedded SDMMC I/O calibration cell provides e.MMC/SD I/Os an output impedance reference to limit the impact of process, voltage and temperature on the drivers output impedance. The impedance control is required at high frequency in order to improve signal quality.

The control and procedure to setup the SDMMC calibration cell is described in the section Secure Digital MultiMedia Card Controller (SDMMC).

Figure 8-3. SDMMC I/O Calibration Cell

The calibration cell provides an input pin SDCAL loaded with a 20 KΩ resistor for 1.8V memories and a 16.9 KΩ resistor for 3.3V memories.

According to the e.MMC specification, the output impedance calibration is mandatory for HS200 mode (1.8V) when it is not for other modes (3.3V).

In addition, according to the SD specification, the output impedance calibration is mandatory for 1.8V signaling when it is not for 3.3V signaling.

Thus, the calibration cell design is oriented to get the highest accuracy under 1.8V.

In case of interfacing which would need to operate under both 1.8V and 3.3V, external devices RZQ and CZQ must get values related to the 1.8V mode. The typical value for CZQ is 22 pF.