C8051F850-C-GM Datasheet, PDF (71/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

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 be found in “AN201: Writing to Flash From Firmware", available from
the Silicon Laboratories web site.
9. Disable interrupts prior to setting PSWE to a '1' and leave them disabled until after PSWE has
been reset to 0. Any interrupts posted during the flash write or erase operation will be serviced in
priority order after the flash operation has been completed and interrupts have been re-enabled by
software.
10. Make certain that the flash write and erase pointer variables are not located in XRAM. See your
compiler documentation for instructions regarding how to explicitly locate variables in different
memory areas.
11. Add address bounds checking to the routines that write or erase flash memory to ensure that a
routine called with an illegal address does not result in modification of the flash.
10.4.3. System Clock
12. If operating from an external crystal-based source, be advised that crystal performance is
susceptible to electrical interference and is sensitive to layout and to changes in temperature. If the
system is operating in an electrically noisy environment, use the internal oscillator or use an
external CMOS clock.
13. If operating from the external oscillator, switch to the internal oscillator during flash write or erase
operations. The external oscillator can continue to run, and the CPU can switch back to the
external oscillator after the flash operation has completed.
Additional flash recommendations and example code can be found in “AN201: Writing to Flash From
Firmware", available from the Silicon Laboratories website.
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