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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 ??.


Port Number: 2Dh

Function: 32768Hz Crystal Control

The TI-83+SE and above have a 32768 Hz crystal, just like the ones found in most wrist watches. When the calculator turns on, the crystal is automatically powered up. This port lets you control what happens when the calculator turns off.

Read Values

  • This port reads the last value written to it.

Write Values

  • Bit 0
    • On the TI-83+SE, setting this bit will cause the calculator to keep the crystal active even when you enter the low-power "off" mode. This is useful because it takes a few milliseconds for the crystal frequency to stabilize after it starts, so you don't have accurate timing until then.
    • On the TI-84+/C/SE, this bit ought to have no effect whatsoever, because the RTC needs the crystal.
  • Bit 1
    • If the crystal is active and this bit is set, then the crystal timers (ports 30h-38h) will continue counting even if you enter low-power mode. If this bit is reset, the crystal timers will be paused when you enter low-power mode and resume automatically when you exit low-power mode, regardless of whether the crystal itself continues oscillating. For some reason, regardless of the value in this port, the crystal timers do not seem to generate interrupts when you HALT, which might be a bug?
  • Bits 2-7 have no effect and no purpose.


TI sets this to 03 on all applicable models. This makes sense: the same crystals are used in wrist watches, which have far less energy available in their batteries, and yet last for years, so clearly there's little to no battery life penalty to keeping the crystal active on the TI-83+SE.