SPRUIE9D May 2017 – May 2024 DRA74P , DRA75P , DRA76P , DRA77P
The CKE pad can be forced to tri-state when the corresponding bit in the CTRL_CORE_SMA_SW_0 register is set to 0x1. For the ddr1_cke pad this is the CTRL_CORE_SMA_SW_0[0] EMIF1_CKE_GATING_CTRL bit and for the ddr2_cke pad the CTRL_CORE_SMA_SW_0[1] EMIF2_CKE_GATING_CTRL bit. This functionality facilitates fast resume by allowing a strong external pull-down resistor to hold the CKE memory pad low thus keeping the SDRAM in self-refresh while the device is completely powered off. Figure 17-51 and the following example sequence provide more details how this functionality can be used:
Figure 17-51 Example for Using the CKE Tri-state FunctionalityIn case the device is powered off but the SDRAM is in self-refresh (as previously described) it must be taken into account that the SDRAM RESET# signal has to be controlled externally to preserve the SDRAM contents. This is needed as ddr1_rst/ddr2_rst signal is not controlled by the CTRL_CORE_SMA_SW_0 register and therefore cannot be used.