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22. NVMCTRL – Nonvolatile Memory Controller

22.6.6 Security Bit

13.11.5 Testing of On-Board Memories MBIST

The DSU implements a feature for automatic testing of memory, also known as MBIST (memory built-in

self test). This is primarily intended for production test of on-board memories. MBIST cannot be operated

from the external address range when the device is protected by the NVMCTRL security bit. If an MBIST

command is issued when the device is protected, a protection error is reported in the Protection Error bit

in the Status A register (STATUSA.PERR).

1. Algorithm

The algorithm used for testing is a type of March algorithm called "March LR". This algorithm is able

to detect a wide range of memory defects, while still keeping a linear run time. The algorithm is:

1.1. Write entire memory to '0', in any order.

1.2. Bit by bit read '0', write '1', in descending order.

1.3. Bit by bit read '1', write '0', read '0', write '1', in ascending order.

1.4. Bit by bit read '1', write '0', in ascending order.

1.5. Bit by bit read '0', write '1', read '1', write '0', in ascending order.

1.6. Read '0' from entire memory, in ascending order.

The specific implementation used as a run time which depends on the CPU clock frequency and

the number of bytes tested in the RAM. The detected faults are:

– Address decoder faults

– Stuck-at faults

– Transition faults

– Coupling faults

– Linked Coupling faults

2. Starting MBIST

To test a memory, you need to write the start address of the memory to the ADDR.ADDR bit field,

and the size of the memory into the Length register.

For best test coverage, an entire physical memory block should be tested at once. It is possible to

test only a subset of a memory, but the test coverage will then be somewhat lower.

The actual test is started by writing a '1' to CTRL.MBIST. A running MBIST operation can be

canceled by writing a '1' to CTRL.SWRST.

3. Interpreting the Results

The tester should monitor the STATUSA register. When the operation is completed,

STATUSA.DONE is set. There are two different modes:

– ADDR.AMOD=0: exit-on-error (default)

In this mode, the algorithm terminates either when a fault is detected or on successful

completion. In both cases, STATUSA.DONE is set. If an error was detected, STATUSA.FAIL

will be set. User then can read the DATA and ADDR registers to locate the fault.

– ADDR.AMOD=1: pause-on-error

In this mode, the MBIST algorithm is paused when an error is detected. In such a situation,

only STATUSA.FAIL is asserted. The state machine waits for user to clear STATUSA.FAIL by

SAM D21 Family

DSU - Device Service Unit

Datasheet Complete

DS40001882D-page 81