We have a design based on MCF54455EVB with:
- MQX 3.7 (MFS, RTCS and SHELL) with Codewarrior for MCUs 10.1.
- MCF54452 MCU at 200Mhz.
- DDR2 SDRAM, MT47H32M16 (8Meg x 16 x 4 Banks) from Micron.
- RESET CIRCUIT, TPS3305-18D from Texas Instruments.
- We are also using other elements like an external flash, etc. (NA)
The connection of the SDRAM memory with the MCF is pretty straight; as defined in the Application Note AN3522 (DDR2 SDRAM in the Coldfire MCF5445x Microprocessor). We have only added a 100Ohm resistor between SCCLK and #SDCLK and a 22Ohm resistor in every port as in MCF54455EVB.
The RESET IC has its reset trigger (#MR) through an AND port, which includes among others the manual reset hardware (resistor, capacitor and a button). The connection between the RESET IC and the MCU has been checked, and it’s ok.
Our problem occurs when we soft reset the board using the reset button. It seems to appear specially, but not only, when soft resetting while MCUs initialization. The board simply hangs after reset and doesn’t respond to any of the subsequent resets, just a power down able us to restore the system functionality.
As said before, this problem generally appears when resetting while MCUs initialization. We have tried also an external reset every 10S (time enough for initializating the MCU) and an internal MCU reset after initialization with the same result: the system eventually hangs.
We have been tracking down the reason and we have discovered that the MCU sometimes doesn’t set down completely the SD_CKE signal, just a little bit about 100mV. Surprisingly, at times this is enough to reset the SDRAM, but normally the SDRAM fails to initialize and this hangs the board.
I have been reading other posts about similar topics and maybe one of them covers the issue we have encountered.:
The problem with this, is that we don't really make a power off sequence, just a reset and then another, so the external oscilator is always powered. And above that, we are suspecting that the SDRAM maybe couldn't be reseated in some states (while writting or reading).