Erase And Program Operations; Unlocking The Flash Control Register - ST STM32F412 Reference Manual

RM0402
Instruction cache memory
To limit the time lost due to jumps, it is possible to retain 64 lines of 128 bits in an instruction
cache memory. This feature can be enabled by setting the instruction cache enable (ICEN)
bit in the FLASH_ACR register. Each time a miss occurs (requested data not present in the
currently used instruction line, in the prefetched instruction line or in the instruction cache
memory), the line read is copied into the instruction cache memory. If some data contained
in the instruction cache memory are requested by the CPU, they are provided without
inserting any delay. Once all the instruction cache memory lines have been filled, the LRU
(least recently used) policy is used to determine the line to replace in the instruction memory
cache. This feature is particularly useful in case of code containing loops.
Data management
Literal pools are fetched from Flash memory through the D-Code bus during the execution
stage of the CPU pipeline. The CPU pipeline is consequently stalled until the requested
literal pool is provided. To limit the time lost due to literal pools, accesses through the AHB
databus D-Code have priority over accesses through the AHB instruction bus I-Code.
If some literal pools are frequently used, the data cache memory can be enabled by setting
the data cache enable (DCEN) bit in the FLASH_ACR register. This feature works like the
instruction cache memory, but the retained data size is limited to 8 rows of 128 bits.
Note: Data in user configuration sector are not cacheable.
3.5

Erase and program operations

For any Flash memory program operation (erase or program), the CPU clock frequency
(HCLK) must be at least 1 MHz. The contents of the Flash memory are not guaranteed if a
device reset occurs during a Flash memory operation.
Any attempt to read the Flash memory on STM32F4xx while it is being written or erased,
causes the bus to stall. Read operations are processed correctly once the program
operation has completed. This means that code or data fetches cannot be performed while
a write/erase operation is ongoing.
3.5.1

Unlocking the Flash control register

After reset, write is not allowed in the Flash control register (FLASH_CR) to protect the
Flash memory against possible unwanted operations due, for example, to electric
disturbances. The following sequence is used to unlock this register:
1. Write KEY1 = 0x45670123 in the Flash key register (FLASH_KEYR)
2. Write KEY2 = 0xCDEF89AB in the Flash key register (FLASH_KEYR)
Any wrong sequence will return a bus error and lock up the FLASH_CR register until the
next reset.
The FLASH_CR register can be locked again by software by setting the LOCK bit in the
FLASH_CR register.
Note: The FLASH_CR register is not accessible in write mode when the BSY bit in the FLASH_SR
register is set. Any attempt to write to it with the BSY bit set will cause the AHB bus to stall
until the BSY bit is cleared.
Embedded Flash memory interface
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