What is ‘Usable Memory’ ?
Every NFC Tag contains a small amount of non-volatile memory which is used to store your data. Non-volatile memory is memory that retains stored information when power is removed. Data for NFC tags is commonly measured in units called bytes and each byte is equivalent to approximately one character of text. However, when writing to an NFC tag there will always be some extra bytes of hidden information which tells your phone what to do. This means that there is a difference between actual memory and usable memory.
How much memory do you need ?
Unfortunately, apart from web addresses (URL) and text, it’s not always quite as simple as five more characters equals five more bytes, but the following table gives you a reasonable idea what you might need.
|Web Address||nfc.today||14 Bytes *|
|SMS||‘Finished work. I’ll be home in 30
minutes !’ to 07777555666
|To – firstname.lastname@example.org
Subject – NFC Help
Message – How much memory does an email use ?
|vCard||Contact Name – Seritag
Contact Number – 02001234567
Address – Seritag, London
Website – nfc.today
|Trigger**||Turn phone to silent||76 Bytes|
|Trigger**||Turn on wifi and launch Netflix app||118 Bytes|
|Trigger**||Wifi on, phone to silent and launch Netflix app||122 Bytes|
** Trigger App for Android.
Note that because of the way the data can be stored, the memory required to store ‘www.nfc.today’ can be the same as ‘nfc.today’. Additionally, https and http take the same amount of memory space. However, there are two different ways to store a web address and if it’s done ‘the other way’, then storing www.nfc.today would take more memory equivalent to the additional characters stored.
NFC chip options
So now you know how much memory you might need, working out which chip you need is simply a case of selecting one with enough usable memory for your data. In general, our preferred option, the NTAG213 chip is the perfect choice.
|NFC Chip||User Memory|
|MIFARE Ultralight / NTAG210||48 Bytes|
|NTAG203 / 213||144 Bytes|
|Broadcom Topaz||454 Bytes|
Using NFC and vCards
In general, NFC.Today strongly recommend not using vCards with NFC. There’s two reasons. Firstly, we think it is always best to consider NFC Tags as a link to the data rather than the data itself. In this ‘internet of things’ world, data is dynamic and changeable. Storing a vCard onto an NFC Tag is static. Use a generic landing page or just connect to your LinkedIn account is a better option. Secondly, while there are standards for vCards, they tend to be interpreted with some flexibility. Which means that a vCard stored by one phone might not look exactly the same on another. If you do really like the idea of vCards, then make sure you use a NTAG216 or Topaz chip because the older ‘1k’ chips are not compatible with all mobile phones.
So what actually is in the ‘hidden information’ ?
Lets break this into three parts. Firstly, data that is ‘hard coded’ onto the NFC chips during manufacture such as the unique ID number. Secondly, some tags, such as the NTAG213, have additional memory bytes allocated to identify how to manage parts of the memory space. Thirdly, there’s always a few bytes used to ‘wrap’ around your data describing what your data is – such as text or a web address.
So what actually is a byte ?
To keep things simple, a byte is composed of eight ‘bits’. A bit is a single binary ‘switch’ – 1 or 0 (think yes or no). So, a byte contains eight 1’s or 0’s, for example 10110101. However, each bit in the byte has an increasing value, like the number 123 can be broken into one hundred, two tens and three. So, ’10’ is actually ‘worth’ 2, and ‘100’ is worth 4. Ultimately, this means that the full ‘11111111’ is worth 255 providing a total combination of 256 numbers (0 inclusive).
In the case of NFC tags, this range of numbers relates to a standardised set of letters and characters. So, the number 114 relates to the letter ‘r’, 115 to the letter ‘s’ and so on.