One advantage of eMMCs (embedded MultiMediaCards) is that they are standardized by the technology standards association JEDEC. Pin layout, register designations and usage, power supply, and controller functions are thus defined and managed memories backward compatible. Each standard update is given a new number, indicating that this eMMC generation supports the features of the previous one plus new and improved features.
From JEDEC 5.0 and higher, the eMMC firmware supports a service health report, which assists with the design process and field maintenance. Similar to the well-known S.M.A.R.T. functionality from SSDs and HDDs, it delivers basic data on the current state of the flash cells within the eMMC. This provides the host with information on the remaining write/erase cycles and the overall state of the eMMC’s flash memory based on the remaining spare blocks.
As such, live information is available about the state of the memory after it has already been used for a certain period of time under certain conditions. This information can serve as the basis for simulating expected field use over many years in the lab and learning how this impacts the longevity of the data.
Thanks to the level of standardization, a design created for an older eMMC version can also be used for the latest generation. The new features or interface options of the younger generation are not available in this case, but all the features of the older one are also implemented into the new generation – ideal for applications with long development cycles.
A potential obstacle may, however, be the driver of the MMC interface on the host. It may initially ask for the JEDEC version of the eMMC and abort if it fails to recognize the input number. This stumbling block can be avoided by updating the driver.
Seeing as the pin assignment is also standardized, developers can freely choose between the various package versions and memory densities. A BGA measuring 11.5 mm × 13 mm is the typical package size for a standard temperature eMMC. Kioxia also offers a smaller 11 mm × 10 mm BGA package for the 4 GB eMMC.
Temperature influences data retention
The first question that needs to be answered when considering the design is: What memory density is required for the data in the customer application? eMMCs are available in capacities of 4 and 128 GB.
It is also essential to take into account the ambient temperature in which the eMMC is to be used. The standard operating temperature range is –25 to +85 °C. For applications where this is not enough, e.g. computer-on-modules (CoMs) that can be used in widely varying environmental conditions or power inverters for solar systems, Kioxia offers eMMCs with an extended temperature range of –40 to +105 °C in capacities of 8 to 64 GB.
However, the temperature range only indicates the operating temperature for the eMMC, not how long the data are retained. If the eMMC is operated frequently over a long period of time at temperatures significantly above 40 °C, users are well advised to talk to the supplier about the individual application. This excludes the possibility of experiencing shorter data retention periods.
Another way to make data more robust to higher temperatures is to use the enhanced user data area, also known as “pseudo SLC” mode. The enhanced user data area is available from eMMC standard 4.4 and higher, thereby making the storage area in question more reliable and powerful. It must be noted, however, that this reduces the available overall density.
The enhanced user data area improves the reliability, performance, and endurance of an eMMC by using only one instead of two bits per cell.
Increasing the data transmission rate
If the application requires a certain data transmission rate, several points need to be considered. It is important to know that there is a correlation between the memory density and the read/write performance of an eMMC. If the application requires a higher data throughput rate than provided by a standard eMMC, the following options are available:
- If mainly the reading speed is to be improved, switching from the HS200 to the HS400 interface is a good idea. However, HS400 is only available from JEDEC 5.0 and higher and requires an additional pin for the interface.
- If mainly the writing speed is to be improved, switching to the enhanced user data area is an efficient approach. Note, however, that this reduces the available level of density.
Determining and extending data retention
Data retention is basically influenced by two factors: the number of write/erase cycles during the service life and the operating temperature. An MLC-based NAND flash memory in an eMMC offers around 3,000 write/erase cycles at 40 °C. Whether this is enough for the expected service life of a product in which the eMMC is used depends heavily on the usage scenario. When calculating the expected service life of the memory, the WAF (write amplification factor) must also be taken into account. Fig. 2 shows the corresponding formula.
The result is, however, not a reliable value but only an approximation, as the actual data retention rate additionally depends on the specific usage of the individual device. If, based on this calculation, it is possible to assume that the service life of the eMMC in the anticipated usage scenario does not correspond to the expected service life of the product, there are two ways to extend it:
- Using the enhanced user data area. This increases the number of available write/erase cycles by a factor of ten compared to the normal mode (at 40 °C). The following also applies in this case: The available density is reduced.
- Selecting a high-density eMMC. The more density available, the greater the area of the memory controller for wear leveling. This means less stress from the write/erase cycles for the individual cells.
eMMCs in production environments
Once the design has been completed, yet another aspect has to be considered in the production process if the data are to be loaded to the memory prior to the reflow or soldering process. At a temperature of approximately 260 °C, the soldering process exposes the NAND cell to extreme stress. This might have a negative impact on data retention or even lead to data loss. To avoid this unwanted scenario, Kioxia has developed a special firmware function. Basically, there are only restrictions regarding the maximum data size that can be processed with the aid of this function.
If these considerations are accounted for during the design process, designers are provided with a durable, high-performance, and reliable memory solution based on eMMC.
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