Oki's ML674K and ML675K series of MCUs have a powerful re-map feature that enables them to re-map different memory banks (ROM, external RAM, internal RAM, external DRAM) to Bank 0 at address 0x00. This feature is facilitated by setting the Remap Control Register (RMPCON).

During software development and debug, it is tempting to try re-mapping by setting this register through a debugger/JTAG connection. Re-mapping is a sensitive task and sometimes re-mapping through the debugger/JTAG connection causes unpredictable behavior such as the JTAG tool (JTAG debugger interface unit) loosing communication with the chip Test Access Port (TAP).

This issue only occurs when one tries to set the register from the debugger through a JTAG connection by either stepping through the code slowly or by setting the register manually. This issue does not occur when running code that contains remap commands in the debugger without stepping or breaking at or near the re-map commands. It also does not occur when the code containing re-map commands is running on the MCU in stand alone mode independent of any JTAG connection.

Thus, to avoid this problem, simply run through the re-map code without breaking or stepping or run this code in stand alone mode.