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Revision as of 07:11, 23 August 2005 by Jim e (Talk | contribs)

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This port only exists as a distinct port on the TI-83 Plus Silver Edition, the TI-84 Plus, and the TI-84 Plus Silver Edition. On the standard TI-83 Plus, it acts as a shadow of port 00.


Port Number: 20h

Function: CPU Speed Port

This port controls the calculator's CPU speed. The CPU can be set to run at either 6MHz or 15MHz with this port.

Read Values

  • 00h : The CPU is running at 6MHz.
  • 01h: The CPU is running at 15MHz.

Write Values

  • 00h : Set the CPU to 6MHz.
  • 01h: Set the CPU to 15MHz.


This port is not available on the normal TI-83+. On the normal TI-83+ this port is a shadow of Port 00h.

This port can contain values of 2 and 3, but the difference in speed is negligible. Using the crystal timers I came to these approximate values.

Port Contents CPU Clock Speed
00 ~6.089 mhz
01 ~14.965 mhz
02 ~14.980 mhz
03 ~14.990 mhz

I suggest still using 1 as the speed setting value since the total effect of the other values are not known. These modes are distinguished by LCD delaying hardware, ports 29-2C.


 in a, (2)
 and 80h
 jp z, NoCPUGoverner
 rlca ;Move the 1 bit in bit 7 to bit 0 (80h -> 01h)
 out (20h), a

Michael Vincent documented another method to set the CPU speed (which is probably faster than my example).

in a, (2)
 sbc a, a
 out (20h), a

The only side effect of this is that on the TI-83+ Basic this will cause both linkport lines to go high - which shouldn't matter too much if you're not using the linkport at that time, especially since both lines are high normally...

Credits and Contributions

  • Michael Vincent: Documentation and his faster approach as found here.