If you looked at the video I posted on previous project log, you saw that I had great difficulty in typing in Basic programs because keyboard repeat rate was just crazy when CPU was running at 15x speed. 

I decided to tackle this problem, by reading the TI ROM code from the excellent book "TI Intern". Page 21 looked promising, there was was some kind of keyboard scanning routine delay:

Time delay routine at >0498
0498 LI 12,>04E2
049C DEC 12
049E JNE >049C
04A0 B *11

Unfortunately changing the above did not help, I modified the counter from >04E2 to >024E2, but this did not help.

After a little more searching (just for the word repeat in the book), I found a more promising piece of code. This time it was not in the Basic ROM, but in Basic GROM. GROM contains code in the interpreted GPL language, not TMS9900 machine code. I don't really know too much about GPL, but hey let's try changing it and see what happens:

Page 149 and 150 talk about repeat counter GPL code. Memory location >830D is set to zero and when it exceeds >FE, repeat occurs. After repeat that location is decremented by >1E (or this is what I think the GPL code is doing). So the next attempt is to change the GPL code
2A6B SUB @>830D,>1E 
to a larger subtract, so that repeat would be slower. This actually helps! But the range is too small and sporadic repeats still occur, even SUB >FE is not enough. The parameter is byte sized, so I cannot subtract more than that. The FPGA CPU just goes too fast and the counter gets incremented from zero to FF too quickly.

Then I got another idea: What if I could disable the repeat code altogether? At 2A4F there is CLR @>830D and it is a two byte opcode, just the same length as the INC opcode at 245F which is taking care of counting the repeat up.

What if we just copy the CLR opcode to 2A5F, overwriting the INC? Then key repeat counter never increments, and we should never get into trouble, right?
2A4F contains 86 0D and this must be CLR @ opcode.
2A5F contains 90 0D and this must be INC @ opcode. So I'll just put 86 in 2A60 and hope for the best. 

That worked! No more repeats and keyboard is usable under TI Basic. The downside of this fix is that while it helps with TI Basic, I don't know if it helps in other programs such as TI Extended Basic, which may use their own code for key repeat - I guess I will see.