Serial EEPROM: Fixed in Place or Removable?

There are many ways of incorporating this popular memory type into your embedded system, but one of the early questions to ask yourself is: permanent installation or have the ability to easily replace the EEPROM?

Electrically erasable programmable read-only memory (E2PROM but more commonly called EEPROM) is one of five common non-volatile memory (NVM) types.

Here’s how they compare:

  • Read only memory (ROM) – Written to using masks, after which erasure is not possible.
  • Programmable read only memory (PROM) – written electrically, after which erasure is not possible.
  • Erasable programmable read only memory (EPROM) – written electrically, after which erasure is possible using UV light (wiping the entire chip).
  • Flash – written electrically, after which erasure is possible (electrically) in blocks.
  • EEPROM – written electrically, after which erasure is possible (electrically) in bytes.

 

Serial versus parallel

Serial EEPROM has three main advantages over parallel EEPROM, including:

  • Lower power consumption (circa one tenth).
  • Fewer I/O pins/pads
  • A smaller physical size.

While serial does not offer the same large capacities or high interface speeds as parallel, these tend not to be requirements for the vast majority of embedded systems.

 

Soldered or removable?

Before embarking on your PCB and enclosure design, it’s worth thinking about just how ‘embedded’ you want the serial EEPROM to be. Do you go down the permanent installation (i.e., soldered to the PCB) route or do you opt for a removable solution?

EEPROM ICs/chips are of course available as plastic dual in-line package (PDIP) and small outline IC (SOIC) package types.

Figure 1 – Above, a Microchip 4Kbit EEPROM in PDIP form factor (on the left) and an Atmel 8kbit EEPROM in SOIC form factor (on the right).

The ICs shown in figure 1 would be mounted on a PCB using plated through-hole (PTH) and surface mount technology (SMT) board population processes, respectively. Should you need to replace the memory at a later date a board rework would be required.

The use of IC holders is an option. Most are like sockets and accept ICs with pins but you can also get ones that accept SMDs; they have lids that hold the devices in place. The holders themselves can be PTH or SMT. Generally, there’s minimal impact on PCB layout or BOM (and therefore cost).

 

Rugged & Secure?

If memory is to be upgraded in the field, a potential problem is risk of damage to the IC, plus the fact you may not have easy access to the IC holder. The ICs can go in in two ways.

One option here is to consider the kind of carry-in-pocket portability that USB memory stick / thumb drives afford. However, USB drives are Flash and have far higher capacities than you probably need for an embedded system.

However, portable drives are available that contain serial EEPROM.

Figure 3 – Above, a Datakey SlimLine® memory token and a PTH PCB-mountable receptacle. Token capacities range from 1Kbit to 64Mbit and devices are available with SPI, I2C or Microwire interfaces.

Though used as if it were a USB drive – i.e., it can be carried in a pocket without fear of bending an IC pin or pad – it has many advantages over a commercial device.

Firstly, it can be inserted in the receptacle either way up, which is of great benefit if you’re accessing the PCB in a confined space. Secondly, the data on the token is more secure than if it were on a USB drive because it the token can only be accessed via a corresponding receptacle; and in the case of the token and receptacle in figure 3, they are only available through authorised channels.

 

On the outside?

Above we spoke about accessing the PCB to change the EEPROM, but if we go down the portable device route, we have the flexibility of having the receptacle somewhere else in the system; and panel mount versions are available.

Figure 4 – Above, a Datakey SlimLine® memory token and panel mount receptacle.

Summary

Serial EEPROM is an extremely popular memory type for the embedded systems engineer. Depending on the application it may be necessary to change the EEPROM to ensure overall system longevity.

The use of IC holders will spare you from board reworks, but you may wish to consider using removable solutions that are more portable, rugged and secure. You could end up with a far more versatile system, and memory upgrades would not necessarily require highly skilled engineers. Indeed, the end user could be sent replacement tokens (for example), with instructions.

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