SDRAM initialization

Question

I've been trying to study how the ARM bootloader works, but initialization of SDRAM is still somewhat a mystery to me. For example AT91 Bootstrap uses following function for initialization. I think I understand most of the steps, but why step #7 (8 auto-refresh cycles) is needed?

int sdramc_initialize(struct sdramc_register *sdramc_config,
                unsigned int sdram_address)
{
unsigned int i;

/* Step#1 SDRAM feature must be in the configuration register */
sdramc_writel(SDRAMC_CR, sdramc_config->cr);

/* Step#2 For mobile SDRAM, temperature-compensated self refresh(TCSR),... */

/* Step#3 The SDRAM memory type must be set in the Memory Device Register */
sdramc_writel(SDRAMC_MDR, sdramc_config->mdr);

/* Step#4 The minimum pause of 200 us is provided to precede any single toggle */
for (i = 0; i < 1000; i++) ;

/* Step#5 A NOP command is issued to the SDRAM devices */
sdramc_writel(SDRAMC_MR, AT91C_SDRAMC_MODE_NOP);
writel(0x00000000, sdram_address);

/* Step#6 An All Banks Precharge command is issued to the SDRAM devices */
sdramc_writel(SDRAMC_MR, AT91C_SDRAMC_MODE_PRECHARGE);
writel(0x00000000, sdram_address);

for (i = 0; i < 10000; i++) ;

/* Step#7 Eight auto-refresh cycles are provided */
for (i = 0; i < 8; i++) {
        sdramc_writel(SDRAMC_MR, AT91C_SDRAMC_MODE_AUTO_REFRESH);
        writel(0x00000001 + i, sdram_address + 4 + 4 * i);
}

/* Step#8 A Mode Register set (MRS) cyscle is issued to program the SDRAM parameters(TCSR, PASR, DS) */
sdramc_writel(SDRAMC_MR, AT91C_SDRAMC_MODE_LOAD_MODE);
writel(0xcafedede, sdram_address + 0x24);

/* Step#9 For mobile SDRAM initialization, an Extended Mode Register set cycle is issued to ... */

/* Step#10 The application must go into Normal Mode, setting Mode to 0 in the Mode Register
 and perform a write access at any location in the SDRAM. */
sdramc_writel(SDRAMC_MR, AT91C_SDRAMC_MODE_NORMAL);        // Set Normal mode
writel(0x00000000, sdram_address);        // Perform Normal mode

/* Step#11 Write the refresh rate into the count field in the SDRAMC Refresh Timer Rgister. */
sdramc_writel(SDRAMC_TR, sdramc_config->tr);

return 0;
}

Answer 1

This is mostly dependant on the SDRAM you are using. For example, take a look at the Application Note TN-46-08: Initialization Sequence for DDR SDRAM.

Also, I have found that the initialization sequence is not so strict. Usually you will stick with whatever is provided already and modify it according to your specific needs. If it doesn't work already, you might want to go into details and check with your SDRAM manufacturer's datasheet

ARM gives an example initialization sequence as well for various architectures, but considers them a "starting point".

Answer 2

Note that there are two separate initializations going on with SDRAM, but their steps often need to be interleaved. On one hand, you need to initialize the SDRAM controller inside the ARM chip, and configure it for the specific connectivity and timing parameters of the SDRAM chip(s) you're using. On the other hand, you also need to configure the SDRAM chip(s) and put them through their initialization sequences, by causing the controller to execute special command cycles on the memory bus.

Step #7 is an example of the latter — it's part of the startup requirements for the SDRAM chip itself.