AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

/* global variables for the MPFFS reader code - extern declarations */

extern struct inode *mpffs_active_index;
extern int mpffs_root_ino;
extern int mpffs_init_done;

extern const u32 mpffs_base_addr;
extern const u32 mpffs_sector_size;
extern const int mpffs_nsectors;