AVR-Chat

Apologies in advance for the thoroughly non-AVR question but I'm really 
tearing my hair out with an LCD problem and I'm sure there are some 
really clever/experienced LCD'ers here (grovel, scrape) ...

We've recently switched to yet another cheaper, brighter, 16x2, HD44780 
compatible LCD (Ampire AC162DY1LY). A small number of these latest 
displays lock-up after correctly initialising and executing a (variable) 
number of display commands. By "lock-up" I mean that the busy-flag stays 
set for at least 150ms, after which time the watchdog resets the micro, 
which then attempts to re-initialise the display, and fails, presumably 
because it's still busy. Display shows the last written text overwritten 
by blocks on the top line, bottom line blank. The display controller 
appears to still be alive because the successive E-cycles show different 
data-bit patterns for the alternate BF and AC3 cycles (4-bit mode).

The LCD driver code has been untouched over 15 years, several thousand 
displays and half-a-dozen different display types. I've double-checked 
the timings and they're not even close to marginal. Code below [ok, it's 
an 80C196 but the comments are in english - 12MHz - port writes take 
833ns, branches 833/1500ns - I'll claim it's a deliberate reminder of 
how what a marvel AVR RISC is].

Has anyone experienced anything like this? Should I suspect faulty 
displays? Or my code?

TIA,
Mark

void LMFWriteData( void ) {
  /* on entry LMDATA contains DB0-7 */
  /* check busy status */
  asm bbc lmNoDisplay,0,checkBusyFlag ;
  asm br lmfwdEnd ;
checkBusyFlag:
  ioport1 = 0x2f ;                  /* rw=1, db4-7=weak1, e=rs=0 */
  ioport1 = 0x6f ;                  /* e=1, db7->busy flag */
  asm bbc ioport1,3,busyFlagClr ;
  ioport1 = 0x2f ;                  /* e=0 */
  ioport1 = 0x6f ;                  /* e=1, db7->AC3 */
  asm br checkBusyFlag ;
busyFlagClr:
  ioport1 = 0x2f ;                  /* e=0 */
  ioport1 = 0x6f ;                  /* e=1, db7->AC3 */
  ioport1 = 0x2f ;                  /* e=0 */
  /* write instr */
  asm ldb LMDATAHI,LMDATA ;
  asm shrb LMDATAHI,#4 ;            /* LMDATAHI.0-3=hi data */
  asm andb LMDATA,#0x0f ;           /* Clr LMDATA.4-7 */
  asm ldb ioport1,#0x1f ;           /* put rs=1,r/w=0,db4-7=weak1,e=0 */
  asm addb ioport1,LMDATAHI,#0x50 ; /* e=1,db4-7=hi data */
  asm addb ioport1,LMDATAHI,#0x10 ; /* e=0 */
  asm addb ioport1,LMDATA,#0x50 ;   /* put lo data,e=1 */
  asm addb ioport1,LMDATA,#0x10 ;   /* e=0 */
lmfwdEnd: ;
}