An other RTC

Description Download ! Documentation Pictures and Schematics History

  The main problem with the DS12887A is its size : you cannot close your TI with it inside and it's almost impossible to have it and the plus module at the same time, without speaking of the TI89. That's why we decided to interface a new chip : the 72423 (whose manual can be found here) which is a very little thing as you can see below.

    Here is a package contaning stuff about interfacing a 72423 RTC to the TI92 bus.
This isn't too much difficult as you could see.

Files contained in the zip:

Source\ clock.asm & clocktsr.asm

Source code to display the clock on-calc

Source\ reg.asm & regtsr.asm

Display the registers of the 72423 chip

Source\ Speed.asm

Needed to be improved, fully bugged. To calculate exactly the speed of the calc thanks to very precise interrupt

Source\ 72423.h

Macros for the RTC


File of the schematic, orcad format


Datasheet of the RTC


guess what

Shematic72423.gif (47629 bytes)

   The even smaller thing you can see on the back of the chip is a 74HC02, a gate we use to invert a signal. The program running on the picture below is a TSR; Like this, we can use the calc normally. You'll need to patch the TIOS if you want the  RTC running well on a TI92+/89... i.e: In AutoInt1 (For TI89/92+ only) the TIOS write the key code at a debug port: $C00000. Actually, for us, it seems that it is really useless, so why couldn't we use this Chip Select ???

Here is the instruction to patch:     move.w     (4,a7),(A0)    A0 pointing to, guess what, $C00000... and in stack you will find the key code .w long. Simply replace this instruction with two nop;

Replace $30AF 0004 with $4E71 4E71, et voilà !

on ROM 2.03: @ to patch: $4A2A90

on ROM 1.01: @ to patch: $49D2AE

on ROM 1.00: @ to patch: $49CB80

72423.gif (2891 octets)

Tiviewsmall.jpg (18978 octets)

Bigzoomsmall.jpg (17200 octets)

Ticlosedsmall.jpg (12992 octets)

Withplusmodulesmall.jpg (10904 octets)

Click on the pictures to enlarge

11/06/2000 -
  • Updated the documentation.
06/12/1999 1.0
  • First release of the 72423 project.

DBA []