[SOLVED] Microchip PIC18F_K Series Battery Powered App. Flash/Code loss/corruption

Hi!

I am using Microchip PIC18F26K22 MCU.

I have a battery powered board. There is a Li-Ion battery cell (because of GSM module). The voltage is from 4.2V to 3.7V in application.
MCU supply voltage is 3V. From battery to MCU, there is LDO chip between them. Also there is a serial schottky diode after battery. Battery voltage is reducing by MCP1726 (adjustable) ldo regulator.
When battery voltage drop to 3.7V level, the LDO voltage is under 3V corresponding by mcu side circuit current load.
When my device leaved alone, the battery voltage is dropping under 3.7V and also MCU voltage is falling to low levels. Example 2.4V. I can't see the action of that situation. Because, these devices are REMOTE telemetry devices.
Returned back devices by users with battery (in shipment, unreleased battery make device run and it see to low mcu voltage levels)

In repairment LAB, effected broken devices DONT start to RUN. We are READING MCU internal FLASH, the code bytes are broken. (first area of code is branch to application codes and interrupt vectors )

My questions:
- What is this problem (by name in society)?
- How can i solve this or protect MCU flash ( BrownOut Reset? WriteProtect? ... )

Thaks for your suggestions.

In battery powered applications, BrownOut reset is most important preporty to protect low voltage levels.
I watch my circuit by simulating battery voltage with Adjustable LDO regulator. The battery voltage is typicaly 3,7V to 4,2V in application. For MCU supply there is a adjustable MCP1726 LDO tı give 2.85V. This voltaje is important for GSM module without using voltage level trasnlaters.
From battery input to MCP1726 output. The voltage drops are SCHOTTKY diode lose and LDO drop out voltage. For 4.2V input, LDO out is perfectly 2,85V. But at low currents, Schottky drop voltage is 0,3V and LDO is request minimum about 0,4V. So, if battery voltage is going under 2,85 + 0,4 + 0,3 ~= 3,6V, output voltage begins to be affected these decrising.
In my application, after LDO output fall under 1.8V, my MCU stops working. I am reading MCU FLASH from MPLAB via ICD3, i see there are some flash block parts become 0xFF.

Becouse of too much time spent, i enabled BOOTSIDE write protection and BOR for 2,7V. There is no problem for now expect GSM MODULE brust current caused voltage drops. It makes BOR reset action. It is shown when battery capacity about %30 and under. It would be solved by incrising BOR voltage level to 2.4V.