EMMC Lifetime (AN50)

Affected Products

All congatec products with eMMC

Preface

This application note explains how to estimate, increase, and monitor eMMC lifetime.

Terminology

Term	Description
eMMC	Embedded Multi-Media Card
OTP	One Time Programmable
ECC	Error Correction Code
P/E	Program-Erase
SLC	Single Level Cell
MLC	Multi Level Cell
TLC	Triple Level Cell
QLC	Quadruple Level Cell
pSLC	Pseudo  Single Level Cell

Introduction to eMMC Lifetime

Embedded Multi-Media Cards (eMMCs) are based on NAND flash. All NAND flash-based storage devices have a limited lifetime.

In simple terms, programming and erasing data gradually weakens a NAND flash cell until it can no longer properly hold a charge or differentiate between the voltage levels. This phenomenon limits the lifetime of an eMMC. A detailed explanation of this phenomenon is outside the scope of this application note.

Flash Controller

The flash controller is part of an eMMC. It improves the lifetime of the NAND flash via various features, such as:

• Wear Leveling Repeated writes to the same memory cell eventually lead to the cell no longer being able to retain data. The wear leveling feature ensures that the data is distributed evenly.
• Bad Block Management A bad block is a flash block that cannot be successfully written or erased without bit-errors. Information about existing bad blocks is stored on the device by the manufacturer. Additional bad blocks can develop over time. The flash controller recognizes them and adds them to the list of bad blocks. Future write/erase procedures can work around these bad blocks.
• ECC Error correction code is used to identify and correct errors. This is typically achieved by adding a number of redundant bits to a data string. The encoded message can then be decoded by the controller to check for and correct bit errors in the message.
• Garbage Collection If data within certain pages of a block is no longer needed, the controller can just read back and rewrite the necessary data to a previously erased empty block. The leftover pages can then be used for new data.

Flash Cell Types

eMMCs implement one main flash cell type. The different types can store different numbers of bits per cell. Flash cell types with a lower number of bits per cell can typically endure a significantly higher number of Program-Erase (P/E) cycles.

The table below compares the different flash cell types. Use the stated P/E cycles as a guideline only and request them from your FAE for a particular congatec product.

	SLC Single Level Cell	3D-MLC Multi Level Cell	3D-TLC Triple Level Cell	3D-QLC Quadrupel Level Cell
Bits/Cell	1	2	3	4
Density	Low	Mid	High	Highest
P/E Cycles	~100,000	~5,000	~3,000	~1,000

Note:

The flash cell types with planar (2D) or 3D NAND technology feature different P/E cycle values. 3D NAND flash increases capacity and reliability of an eMMC.

Enhanced User Area and pSLC Mode

The JEDEC JESD84-B51 standard describes a storage mode, whereas all or parts of the eMMCs memory can be reconfigured to a more robust and reliable mode.

How the enhanced user area is implemented is up to the eMMC manufacturer.

For eMMCs that are used on congatec products, the enhanced user area is pseudo-SLC mode.

This mode greatly increases the maximum number of P/E cycles by reducing the bits per cell to one, same as SLC. That’s why this mode is called pseudo-SLC.

The trade-off is a greatly reduced storage capacity – it is divided by the bits per cell of the flash cell type (e.g., leaves 1/3 of the original storage capacity of an eMMC with TLC flash cell type).

To enable this mode, refer to section 3.2.

Note:

The enhanced user area mode for eMMCs on congatec products is pSLC.

Estimating the eMMC Lifetime

The amount of logic data that can be written to the eMMC until the end of its lifetime can be estimated with this formula:

TBW = DC * EF / WAF

TBW	Total Bytes Written throughout the lifetime of the eMMC.
DC	Device Capacity in bytes.
EF	Endurance Factor. Use the max. P/E cycles of the eMMC. Request it from your FAE for a particular congatec product.
WAF	Write Amplification Factor. Basically, NAND flash organizes data in groups. Data can only be programmed in groups (pages) and erased in groups of pages (blocks).  A page can be several KB or even MB in size and a block can consist of hundreds of pages. This typically means that the actual amount of physical data written to the NAND flash is a factor of 8 to 20 times greater than the logic data written by the host. This phenomenon is called Write Amplification (WA). A more detailed description of this phenomenon is beyond the scope of this application note.

Divede the TBW by the average logic data written to estimate the lifetime in number of days.

Example

In this example, we estimate the lifetime of a conga-SA5/i-E3950-4G eMMC32 (Revision B.3). This module uses an eMMC with 3D TLC NAND flash technology that supports 3,000 P/E cycles. We assume an inefficient WAF of 20 for our specific use case.

Filling in these values into the previously mentioned formula gives us the following TBW:

TBW = 32 GB * 3,000 / 20 = 4.8 TB

If we assume that 1 GB of logic data is written to the eMMC on average per day, we can estimate the lifetime in days:

Lifetime = 4,8 TB / 1 GB / day = 4800 days or 13.15 years

Note:

Request the P/E cycle value from your FAE for a particular congatec product.

Increasing the eMMC Lifetime

The sections below provide some general guidelines and special procedures that can increase the lifetime of eMMCs. Section 3.2.1 describes prerequisites that apply to several procedures.

General Guidelines

The formula in section 2 introduced variables that influence the lifetime of all eMMC devices. Based on these variables, the following general guidelines can be deduced:

1. Use a congatec product with a larger device capacity (DC)
2. Decrease the amount of logic data Adjust applications to decrease the writing frequency and only writing necessary data. Outsource frequently changed data to a different medium whenever possible (i.e. logs).
3. Decrease the amount of physical data (WAF)

Adjust applications to write more efficient, bigger sequential writes to the eMMC instead of smaller randomized writes. The ideal size depends on the page sized. Another way is described in section "Enabling eMMC Cache"

Enabling Enhanced Mode (pSLC)

This section explains how to enable the enhanced mode. As explained is section "Enhanced User Area and pSLC Mode", the pSLC enhanced mode can be enabled on some eMMCs to greatly increases their lifetime at the expense of storage capacity.

Note:

The procedure was tested on Ubuntu 2.04.1 LTS with kernel 5.8.0-38-generic.

Prerequisites

Follow the instructions below to prepare for enbaling the pSLC mode:

1. Install a Linux based operating system on a medium that is not the target eMMC
2. Install mmc-utils

sudo apt-get install mmc-utils

3. List all available eMMC devices

dmesg | grep mmc Example console output: […] mmc1: new HS400 MMC card at address 0001 mmcblk1: mmc1:0001 M5232 28.5GiB

4. Identify your target eMMC disk name from the output (e.g. mmcblk1)
5. Ensure that the target eMMC is of standard 5.0 or higher with extended CSD register revision 1.7 or higher

sudo mmc extcsd read /dev/yourtargetdiskname | head -n 3

Example console output:

==========================================

Extended CSD rev 1.7 (MMC 5.0)

==========================================

Note:

Replace "yourtargetdiskname" with your target eMMC disk name in all commands.

If these two conditions are met, the pSLC mode can be enabled on the target eMMC.

Enabling Enhanced Mode (pSLC)

Caution:

The procedures described in this section erase all data on your target eMMC and permanently decrease its storage capacity. This procedure is non-reversible (OTP).

Note:

The procedure described in this section can also be used to enable pSLC on one or more partitions. The storage capacity of the other partitions is not reduced. This way, write-intensive tasks may be relocated to a pSLC partition while other partitions can keep their original (larger) storage capacity for less write-intensive tasks.

Replace "yourtargetdiskname" with your target eMMC disk name in all commands.