C8051F970-A-GM Datasheet, PDF (68/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
10.3. Non-volatile Data Storage
The flash memory can be used for non-volatile data storage as well as program code. This allows data such as
calibration coefficients to be calculated and stored at run time. Data is written using the MOVX write instruction and
read using the MOVC instruction. Note: MOVX read instructions always target XRAM.
10.4. Flash Write and Erase Guidelines
Any system which contains routines which write or erase flash memory from software involves some risk that the
write or erase routines will execute unintentionally if the CPU is operating outside its specified operating range of
supply voltage, system clock frequency or temperature. This accidental execution of flash modifying code can
result in alteration of flash memory contents causing a system failure that is only recoverable by re-flashing the
code in the device.
To help prevent the accidental modification of flash by firmware, hardware restricts flash writes and erasures when
the supply monitor is not active and selected as a reset source. As the monitor is enabled and selected as a reset
source by default, it is recommended that systems writing or erasing flash simply maintain the default state.
The following guidelines are recommended for any system which contains routines which write or erase flash from
code.
10.4.1. Voltage Supply Maintenance and the Supply Monitor
1. If the system power supply is subject to voltage or current "spikes," add sufficient transient protection
devices to the power supply to ensure that the supply voltages listed in the Absolute Maximum Ratings
table are not exceeded.
2. Make certain that the minimum supply rise time specification is met. If the system cannot meet this rise
time specification, then add an external supply brownout circuit to the RST pin of the device that holds the
device in reset until the voltage supply reaches the lower limit, and re-asserts RST if the supply drops
below the low supply limit.
3. Do not disable the supply monitor. If the supply monitor must be disabled in the system, firmware should be
added to the startup routine to enable the on-chip supply monitor and enable the supply monitor as a reset
source as early in code as possible. This should be the first set of instructions executed after the reset
vector. For C-based systems, this may involve modifying the startup code added by the C compiler. See
your compiler documentation for more details. Make certain that there are no delays in software between
enabling the supply monitor and enabling the supply monitor as a reset source. Code examples showing
this can be found in “AN201: Writing to Flash From Firmware", available from the Silicon Laboratories web
site. Note that the supply monitor must be enabled and enabled as a reset source when writing or
erasing flash memory. A flash error reset will occur if either condition is not met.
4. As an added precaution if the supply monitor is ever disabled, explicitly enable the supply monitor and
enable the supply monitor as a reset source inside the functions that write and erase flash memory. The
supply monitor enable instructions should be placed just after the instruction to set PSWE to a 1, but before
the flash write or erase operation instruction.
5. Make certain that all writes to the RSTSRC (Reset Sources) register use direct assignment operators and
explicitly DO NOT use the bit-wise operators (such as AND or OR). For example, "RSTSRC = 0x02" is
correct. "RSTSRC |= 0x02" is incorrect.
6. Make certain that all writes to the RSTSRC register explicitly set the PORSF bit to a '1'. Areas to check are
initialization code which enables other reset sources, such as the Missing Clock Detector or Comparator,
for example, and instructions which force a Software Reset. A global search on "RSTSRC" can quickly
verify this.
10.4.2. PSWE Maintenance
7. Reduce the number of places in code where the PSWE bit (in register PSCTL) is set to a 1. There should
be exactly one routine in code that sets PSWE to a '1' to write flash bytes and one routine in code that sets
PSWE and PSEE both to a '1' to erase flash pages.
8. Minimize the number of variable accesses while PSWE is set to a 1. Handle pointer address updates and
loop variable maintenance outside the "PSWE = 1;... PSWE = 0;" area. Code examples showing this can
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