Only the Ti-89 (and the rarely seen 92 / Voyage series) use a 68k.

Ti-86 and below all use Z80s. Although the 83 premium CE (and the 84 Plus CE) uses a pretty monstrous (compared to the original Z80s) 48MHz eZ80.

I think it's 68k for the TI-89 and up, and Z80 for the lower models.

The eZ80, I believe, which is backwards compatible but clicked higher and with 24-bit arithmetic.

The headline says TI-83 Premium. It has a Zilog EZ-80, which is basically a hopped up Z80. As far as I can tell, all of the TI-83 models have Z80's.