83Plus:Ports:20

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.

Synopsis

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.

Comments

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 aproximate vaules.

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

I suggests still using 1 as the speed setting vaule since the total effect ofther vaules are not known. These modes are distiguished by LCD delaying hardware, ports 29-2C.

Example

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

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

in a, (2)
 rla
 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.