TBLRD Requires NVMREG Value to Point to Appropriate Memory

The affected silicon revisions of the PIC18(L)F24/25K40 devices improperly require the NVMREG[1:0] bits in the NVMCON register to be set for TBLRD access of the various memory regions. The issue is most apparent in compiled C programs when the user defines a const type and the compiler uses TBLRD instructions to retrieve the data from Program Flash Memory (PFM). The issue is also apparent when the user defines an array in RAM for which the compiler creates start-up code, executed before main(), that uses TBLRD instructions to initialize RAM from PFM.

Work around

Assembly code:

Set the NVMREG[1:0] bits to select the appropriate memory region before executing TBLRD instructions.

C code:

Create an assembly file named powerup.as and include this file with the other files in the project. This file will change the NVMREG[1:0] bits to point to program Flash before any code is executed. Contents of the powerup.as file:

#include <xc.inc>
        GLOBAL    powerup, start
        PSECT     powerup, class=CODE, delta=1, reloc=2
powerup:
        BSF       NVMCON1, 7 
        GOTO      start
        end

If there is a need to change the NVMREG[1:0] value to anything other than ‘10’ and the Interrupt Service Routine uses constants or literal strings, then interrupts must be disabled before the change and restored to ‘10’ before interrupts are enabled.

Affected Silicon Revisions

A3	A4
X