SDRAM devices require periodic calibration of output driver impedance and ODT values to minimize variations in voltage and temperature. This calibration is known as ZQ calibration. The DDR controller includes full support for ZQ calibration, including automatic calibration during the memory initialization sequence and periodic automatic calibrations after initialization.
ZQ calibration can be initiated either automatically or manually. The DDR Configurator provides an option to enable automatic ZQ calibration and to set automatic ZQ calibration period.
Depending on the type of SDRAM memory used in the device, four kinds of ZQ calibration are supported: ZQ calibration long (ZQCL), ZQ calibration short (ZQCS), ZQ calibration reset (ZQRESET), and ZQ calibration initialize (ZQINIT).
The following table lists the types of ZQ calibration commands issued in various SDRAM memory devices.
| Memory | Calibration Type | Time the ZQ Command is Issued |
|---|---|---|
| DDR3, DDR4 | ZQCL | During initialization sequence. |
| ZQCS | Periodically, after initialization (ZQCS is enabled by default but ZQCL can be selected for periodic ZQ calibration) periodic calibration is optional for DDR3. | |
| LPDDR3 | ZQCL | During initialization sequence. |
| ZQCS | Periodically, after initialization (ZQCS is enabled by default but ZQCL can be selected for periodic ZQ calibration). | |
| ZQRESET | When ZQ must be reset to default output impedance. |
PolarFire and PolarFire SoC devices support I/O calibration requests from the Fabric DDR controller, and calibrate only the I/Os that are used with the Fabric DDR controller.