Can you lock only part of a tag?
Yes, on most chip families. Two kinds of lock bits exist: one set covering the first pages of memory, and another covering the rest in blocks. That lets you freeze part of a tag permanently while leaving the remainder writable, which is occasionally exactly what a deployment needs.
Two kinds of lock bits
The first set is small and fixed, covering the earliest pages of memory. These are the bits a tool sets when you tick a lock option, and they are the reason a locked tag is fully frozen.
The second set covers the remaining memory in blocks rather than pages, and exists precisely so that regions can be frozen independently.
Both are one way. Neither set can be cleared, and both are themselves stored in memory that becomes read only once used, so a lock bit cannot be unlocked by locking something else.
How fine the control is
Coarser than you would like. The second set typically covers several pages at a time, so you cannot freeze an individual byte or a single record.
The practical granularity is therefore a region rather than a field, and the exact block size differs by chip family. The datasheet is the only authority for a given reference.
This means partial locking works well for a layout designed around it, and poorly as an afterthought on content that was laid out without thinking about block boundaries.
When partial locking is useful
An identifier that must never change alongside content that must. An asset tag whose serial number is frozen while its maintenance data is updated is the clearest example.
A header your own software relies on, protected against a well meaning colleague rewriting the tag with a generic tool.
And a two stage deployment, where a manufacturer writes and freezes part of a tag before a customer writes the rest.
What these share is a designed layout. Partial locking is a production technique rather than something to reach for casually.
Where it goes wrong
Freezing a region that includes the structure. If the capability container falls inside the locked range, the tag can never be reformatted, which turns any structural damage into a dead tag.
Assuming a tool understands your intent. Most tools expose locking as one checkbox that sets the first bits and freezes everything. Selective locking generally requires addressing the configuration pages directly, with the datasheet open.
And forgetting that the boundary is permanent. If your content later grows past the unlocked region, there is no way to reclaim the frozen part.
Frequently asked questions
Does the lock checkbox in my tool lock everything?
Almost always yes. Selective locking normally requires writing configuration pages directly rather than using a single option, so assume a checkbox means the whole tag.
Can I lock the identifier only?
The identifier is already read only from the factory, so there is nothing to lock. What people usually mean is freezing their own serial written into the content, which is exactly what partial locking is for.
How large is each lockable block?
It varies by chip family and is documented in the datasheet. Assume several pages rather than a single byte, and design the memory layout around the boundaries.
Can I unlock one region and keep another locked?
No. Every lock bit is one way, so the only direction available is locking more of the tag, never less.