AM335x DDR3 initialization / timing violation

[Matthias Feser]

Hi, 

we are using the AM3352 processor in combination with a single 512MB Micron DDR3 chip running the barebox bootloader in our products for several years now. A minor amount of boards (around 5 of 2000) fail the production test, because they do not boot properly after warm reset. In such cases the MLO is loaded, initializes the EMIF and then crashes after a certain amount of accesses to the DDR3. After a cold reset all of these boards run stable and produce no errors when running a deep RAM test. 

I am currently in discussion with a TI employee about this topic. He told me that the bootloader should detect a warm reset and EMIF should not be reinitialized, because DDR3 is automatically put into self-refresh on warm reset. So far he hasn’t told me what the desired init sequence actually is. >From what I have observed while debugging, at least the EMIF clock has to be enabled and CKE brought high. Unfortunately the TRM does not give guidance about this.

Our board initialization code is very similar to other AM335x based boards like beaglebone (400MHz DDR clock), which effectively always initializes the EMIF in the same way by calling am335x_sdram_init(), no matter if cold or warm reset has brought up the processor. From investigating the signals DDR_RESET and DDR_CKE with an oscilloscope, I can tell that even with this same init code the hardware behaves differently in both reset cases.

On cold reset both DDR_RESET and DDR_CKE remain low until initialization, and there is a delay of roughly 436us between the rising edges of DDR_RESET and DDR_CKE. After warm reset DDR_RESET is high and DDR_CKE is low. EMIF initialization results in a short pulse on DDR_RESET with 5us low phase and there is only about 38us delay between the rising edges of DDR_RESET and DDR_CKE.
Both cases violate the DDR3 specification, according to which the delay between the rising edges of DDR_RESET and DDR_CKE has to be 500us min.

In am33xx_config_sdram() a value of 0x2800 is written to EMIF4_SDRAM_REF_CTRL. TI recommends a value of 0x3100 during initialization, which is used in u-boot EMIF initialization code and does not violate the DDR3 specification. I think barebox EMIF init code requires some revision.

I also wonder why EMIF4_SDRAM_CONFIG, REF_CTRL and REF_CTRL_SHADOW are accessed twice when regs->zq_config is not zero (see code snippet below taken from barebox 2022.12.0). Is there any reason behind this?

                if (regs->zq_config) {
                               /*
                               * A value of 0x2800 for the REF CTRL will give us
                               * about 570us for a delay, which will be long enough
                               * to configure things.
                               */
                               writel(0x2800, emif4 + EMIF4_SDRAM_REF_CTRL);
                               writel(regs->zq_config, emif4 + EMIF4_ZQ_CONFIG);
                               writel(regs->sdram_config, CM_EMIF_SDRAM_CONFIG);
                               writel(regs->sdram_config, emif4 + EMIF4_SDRAM_CONFIG);
                               writel(regs->sdram_ref_ctrl, emif4 + EMIF4_SDRAM_REF_CTRL);
                               writel(regs->sdram_ref_ctrl, emif4 + EMIF4_SDRAM_REF_CTRL_SHADOW);
                }

                writel(regs->sdram_ref_ctrl, emif4 + EMIF4_SDRAM_REF_CTRL);
                writel(regs->sdram_ref_ctrl, emif4 + EMIF4_SDRAM_REF_CTRL_SHADOW);
                writel(regs->sdram_config, emif4 + EMIF4_SDRAM_CONFIG);


As soon as I get an update from TI about the proper EMIF initialization after warm reset, I will let you know.

Best regards


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