NVMC — Non-volatile memory controller

When writing is enabled, in CONFIG register for secure region, or in CONFIGNS register for non-secure region, flash is written by writing a full 32-bit word to a word-aligned address in flash.

Secure code has access to both secure and non-secure regions, by using the appropriate configuration of CONFIG and CONFIGNS registers. Non-secure code, in contrast, has access to non-secure regions only. Thus, non-secure code only needs CONFIGNS.

The NVMC is only able to write '0' to erased bits in flash, that is bits set to '1'. It cannot write a bit back to '1'.

As illustrated in Memory, flash is divided into multiple pages. The same address in flash can only be written n_WRITE number of times before a page erase must be performed.

Only full 32-bit words can be written to flash using the NVMC interface. To write less than 32 bits to flash, write the data as a word, and set all the bits that should remain unchanged in the word to '1'. Note that the restriction about the number of writes (see above) still applies in this case.

The time it takes to write a word to flash is specified by t_WRITE. If CPU executes code from flash while the NVMC is writing to flash, the CPU will be stalled.

Only word-aligned writes are allowed. Byte or half-word-aligned writes will result in a bus fault.

When secure region erase is enabled (in CONFIG register), a flash page can be erased by writing 0xFFFFFFFF into the first 32-bit word in a flash page.

Page erase is only applicable to the code area in the flash and does not work with UICR.

After erasing a flash page, all bits in the page are set to '1'. The time it takes to erase a page is specified by t_ERASEPAGE. The CPU is stalled if the CPU executes code from the flash while the NVMC performs the erase operation.

See Partial erase of a page in flash for information on splitting the erase time in smaller chunks.

When non-secure region erase is enabled, a non-secure flash page can be erased by writing 0xFFFFFFFF into the first 32-bit word of the flash page.

Page erase is only applicable to the code area in the flash and does not work with UICR.

After erasing a flash page, all bits in the page are set to '1'. The time it takes to erase a page is specified by t_ERASEPAGE. The CPU is stalled if the CPU executes code from the flash while the NVMC performs the erase operation.

User information configuration registers (UICR) are written in the same way as flash. After UICR has been written, the new UICR configuration only takes effect after a reset.

UICR is only accessible by secure code. Any write from non-secure code will be faulted.

In order to lock the chip after uploading non-secure code, a simple sequence must be followed:

1. Block access to secure code by setting UICR register SECUREAPPROTECT to protected
2. Use the WRITEUICRNS register, via non-secure debugger, in order to set APPROTECT (APPROTECT is automatically written to 0x00000000 by the NVMC)

UICR can only be written n_WRITE number of times before an erase must be performed using ERASEALL.

The time it takes to write a word to the UICR is specified by t_WRITE. The CPU is stalled if the CPU executes code from the flash while the NVMC is writing to the UICR.

When erase is enabled, the whole flash and UICR can be erased in one operation by using the ERASEALL register. ERASEALL does not erase the factory information configuration registers (FICR).

This functionality can be blocked by some configuration of the UICR protection bits, see the table NVMC protection (1 - Enabled, 0 - Disabled, X - Don't care).

The time it takes to perform an ERASEALL command is specified by t_ERASEALL. The CPU is stalled if the CPU executes code from the flash while the NVMC performs the erase operation.

This chapter describes the different protection mechanisms for the non-volatile memory.

An instruction cache (I-Cache) can be enabled for the ICODE bus in the NVMC.

See Memory map for the location of flash.

A cache hit is an instruction fetch from the cache, and it has a 0 wait-state delay. The number of wait-states for a cache miss, where the instruction is not available in the cache and needs to be fetched from flash, depends on the processor frequency, see CPU parameter W_FLASHCACHE.

Enabling the cache can increase the CPU performance and reduce power consumption, by reducing the number of wait cycles and the number of flash accesses. This depends on the cache hit rate. Cache draws current when enabled. If the reduction in average current due to reduced flash accesses is larger than the cache power requirement, the average current to execute the program code is reduced.

When disabled, the cache does not draw current and its content is not retained.

It is possible to enable cache profiling to analyze the performance of the cache for your program using the register ICACHECNF. When profiling is enabled, registers IHIT and IMISS are incremented for every instruction cache hit or miss respectively.