Types of Memory in RFID Tags
RFID UHF Tags Class 1 Gen 2 have a standardized and precise memory structure. RFID UHF chips by world leading manufacturers, including the ones of NXP UCODE and Impinj Monza series, follow this standard.
Here are the 4 types of memory that the Gen2 chips are equipped with.
Please note that the fourth one, the user memory, is not present in all chips. Also, with the exception of reserved memory, the other three may have different sizes depending on the chip. To know the exact size, you can refer to the technical data sheet of the specific RFID Tag, or consult the technical specifications of the RFID chips.
Now let's see the 4 types of memory in detail.
1. Reserved memory
This memory bank contains the kill password and the access password (32-bit each). By default, both passwords have a value of '0', so they are disabled.
The kill password permanently disables the Tag (kills it), that is, it makes the Tag useless. This is a rarely used function, but still useful in some sectors, such as retail: often the RFID Tag is disabled during check out.
The access password can be used to lock and unlock the Tag's writing capabilities. Using the access password, the Tag can assume one of the following 4 states:
- Unlocked
- Perma-unlocked (can never be locked)
- Locked
- Perma-locked (can never be unlocked)
The reserved memory is the only one that can be locked both in Read and Write modes. The other types of memory only support the Write mode lock. This means that even if a Tag is locked, everyone can read it.
2. EPC memory
This memory bank is designed to contain the EPC code, or Electronic Product Code, the GS1 standard that uniquely identifies retail products. That said, the EPC memory can be used freely, for programming personal codes, service numbers, inventory numbers, serial numbers and so on.
EPC memory has a minimum of 96 bits of writable memory. EPC memory is typically used in most applications if they only need 96 bits of memory. There are some Tags which have the ability to allocate more bits to EPC memory from user memory. EPC memory is the first writable memory bank.
Depending on the chip's type, the EPC may have different factory settings. Some manufacturers supply the EPC programmed with the same code, others with a pseudo-random code, others leave it blank. If you want to use the factory set EPC, check how it is provided.
3. TID memory
This memory is used only to store the unique ID number of the Tag. The TID (Tag Identifier) is assigned by the manufacturer when the chip is produced. Generally, this memory is not editable.
Since the initial part of the TID identifies the company that produces the chip, each manufacturer has its own set of codes, and in general the TID code is globally unique.
If the need is to have a unique code to identify things or people, using the TID is the simplest solution, as it saves the EPC programming step.
4. User memory
If you need more memory than the one stored in the EPC section, some chips have extended user memory which can store more information. When it comes to user memory, there is no standard on the number of memory bits that can be written to each Tag.
One of the most popular chips with good user memory is the Monza 4QT, with 512 bits. However, there are some RFID Tags with much larger user memory, up to 4 or 8 kbit. This is the second writable memory bank for RFID chips Gen 2.