Bringing SDRAM in power down mode with LPC1788

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by FutureDesign on Fri Feb 06 07:50:47 MST 2015
We are developing a battery operated LPC1788 device that has a LCD, SDRAM, NOR FLASH, UART and  touch controller.
During the idle state the system has to go to low power mode to preserve the battery.
Among all the devices we find some issues while bringing down the SDRAM to power down state.

As I understand, there is a particular type of CKE signal transition that has to be synchronous with clock that takes the SDRAM device into power down mode.
The user manual of the LPC1788 explains about the method of SDRAM low power mode. It is available in Rev 3 user manual on page 179, section 9.6.1.

The section of the user manual says to set DP, CS and CE bit of EMDDynamicControl register.

However, in LPC1788 EMC register description there is a register called " Dynamic Memory Control Register" that contains CS and CE bit but there is no DP bit.

Investigating on ARM resources we found out that bit # 13 of the DynamicControl register is the DP bit. But Bit # 13 of that register in  LPC1788  is labeled as reserved.

Even though it is labeled as reserved, we tried to set these bits and we seems to be able to bring SDRAM in power down mode. But the LCD DE signal disappears after clearing back  the DP bit in order to use the SDRAM and hence the LCD does not work. During many trial, LCD worked few times but we could not understand why it does not work and rarely works.
SDRAM seems to be working fine after DP bit is cleared.

Our Questions:

* Is it valid to use the DP bit in LPC1788? If so why it disturbs the LCD functionality. Is there any way out for this?
* If DP bit is not suggested for use, then what is the method by which we can take the external SDRAM in power down mode.