Managed NAND

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Raw NAND and TI ROM Bootloaders (RBLs)

Managed NAND has been recommended as a possible alternative to raw NAND. This article discusses and compares raw NAND versus managed NAND support on TI bootloaders and suggests recommendations. Two main challanges while booting from a raw NAND:

Even if RBL is able to identify NANDs, the ECC support in the hardware/RBL can't be changed. All of our current devices have <=4bit/512bytes ECC support in the hardware. MLC NANDs are already requiring >=8bit/512bytes ECC support. There are SLC devices with 1/4bit ECC requirement, but SLC devices are costlier than MLC devices.

Managed NANDs

Managed NANDs are RAW NANDs combined with a memory controller and can support popular interfaces such as MMC (eMMC, moviNAND), SD (eSD) and SPI. The memory controller hides the details of the NAND and provides the intended interface and ECC support. This means that a eMMC/eSD NAND can talk to a embedded TI device through MMC/SD controller.
The limitation arises because the latest eMMC/eSD standard in the market is ahead of what is supported currently in TI devices. The following table lists the IP support in TI DM/OMAP devices.

Devices	MMC	SD

DM355	3.31	1.1
DM6446	3.31	1.1
DM365	3.31	1.1
DM6467	NA	NA
DM6437	NA	NA
DM648	NA	NA
AM3505/17, OMAP35x, AM/DM37x	4.2	2.0
OMAPL137/C6747/C6745/C6743/AM17x	4	1.1
OMAPL138/C6748/C6746/C6742/AM18x	4	1.1
C6A8xx	4.4	2.1

Performance comparison

The table below compares performance throughput (both read and write) for different regular SD cards and managed NAND (eMMC/eSD) cards on DM355 Rev C silicon EVM for three different filesystems EXT2, EXT3 and VFAT mounted on SD/eMMC/eSD cards.
The kernel used to test this is from PSPv3.01 Phase 2 Release – 2.6.32-rc1 kernel.
The filesystem used is Arago filesystem that can be downloaded from link above.
Note that managed NAND performance is comparable with SD and SDHC cards.
More detailed results are available on request (please contact your local sales team for support).

Storage Device	File System	Read (Mbytes/s)	Write (Mbytes/s)	Comment
Raw NAND	JFFS2	5.9	1.97	Buffersize=1024K; 0.8 when buffersize if 100K and 256K
SD	EXT2	8.5	3.7	Using Sandisk 2G card
SDHC	VFAT	11	8.76	Using Sandisk 16G card
eMMC	EXT2	9.87	4.19	Samsung moviNAND 8GB KLM8G4DEFD-A301
eMMC	EXT3	9.84	3.63	Samsung moviNAND 8GB KLM8G4DEFD-A301
eMMC	VFAT	9.97	6.39	Samsung moviNAND 8GB KLM8G4DEFD-A301
eSD	EXT2	10.27	3.54	SanDisk 4GB SDIN2C2-4G
eSD	EXT3	9.09	4.36	SanDisk 4GB SDIN2C2-4G
eSD	VFAT	10.22	7.24	SanDisk 4GB SDIN2C2-4G

Support in UBL,uBoot and Linux Kernel

To support Managed NAND after boot, all the software components (UBL,uBoot and Linux Kernel) should have MMC/SD boot support. Currently UBL and uBoot doesn't have the required support. Latest kernel, 2.6.32-rc1 from PSPv3.01 Phase 2 has MMC/SD support. However, there are few cards that are not recognized by the kernel correctly as is evident from SD/SDHC/eMMC/eSD Performance comparison above.

Is Managed NAND a good solution?

The following table bring us comparison between RAW and Managed NAND.

Features	Raw NAND(MLC)	Managed NAND

Nand Interface Issues	Yes	No
ECC required	Yes	No
MMC/SD interface	No	Yes
Speed	z	~2z
Special software support	No	Yes
Advanced features	No	Yes

Recommendation

1. If price is not a driving factor, SLC NANDs will be supported in most of the current devices for atleast a couple of years.

2. If the device has MMC/SD support, eMMC/eSD is a reasonable solution.

3. If the device with a DSP doesn't have a MMC/SD support (DM648, DM6437, DM6446), and price is a driving factor (to rule out SLC NANDs), software solution for ECC should be considered.