C128 problems

One problem with the MIDI interface for C128 in C64 mode is the glitch on IO2. There is a short glitch every 16 ms, looks like this:

It is some 12 ns long, zoomed with the yellow trace the analog input of the same IO2 signal:

On the C64 there is no glitch, at least not this regular.

Another problem: when I write data with zero page indexed ("sta ($14),y"), IO2 is asserted twice. An example writing $40 to $df00:

But this problem is the same on the C64. If I write with "sta $df00", it look like this:

Adding a NAND gate for the 74LS273 CLK input filtered the glitch, with this function: CLK = (IO2*IO2)*S02 (with * = NAND). And with the new filter, the rising edge of CLK for the latch is even earlier than the rising edge of IO2, which means the data latching should be more safe. This is when I write 0 to $df00 with the NAND filter:


23. Januar 2014, Frank Buß