Enter the TI-GROMmy, a modern replacement board for the system GROMs of a TI-99/4A. It is flash programmable from the 99/4a and can contain multiple grom binaries and plug into your console directly in the grom sockets.
Uses STM32G070KB6 MCU.
- 128K Flash
- 36K SRAM
- 32-pin LQFP
Installation
The grommy2 boards are shipped preprogrammed.
For installation, please refer to the pictures in the presentation.
the process is simple: remove all three system GROM chips, and plug the grommy2 board in their place.
The pictures show installation to the two leftmost GROM sockets. That’s how I have also installed the boards.
However, since the GROM chip sockets share all pins, it is also possible to plug the board into the two rightmost sockets. The only reason why this would make sense is if the contacts are somehow bad on the two leftmost sockets, or they are damaged or something like that.
After installation, and make sure all pins on the bottom side are in the socket and the alignment is good. I always look from the side to make sure that’s the case, see the circled area in the picture below.
Power on the console. It should boot up normally. The red LED will be lit when the grommy2 is active. During normal boot with default console GROM code the LED will be lit almost all the time, since the console is executing GPL code from GROMs almost all the time.
Option switches
If you want to install switches to select options, please refer to the page 4 of the presentation. It shows two signals of interest. All signals mentioned below are available on the side connector of the grommy2, the right angle pin header pictured above.
- PA3 – /BANK1, when brought low, chooses the GROM bank #1 during the boot process. Bringing it low means connecting it to Vss on the same connector. If you want to have a switch for this purpose, connect it between PA3 – /BANK1 and Vss.
- PA2 – /FAILSAFE, when brought low, chooses the failsafe mode during the boot process. Bringing it low means connecting it to Vss on the same connector. If you want to have a switch for this purpose, connect it between PA2 – /FAILSAFE and Vss.
- Vss on the grommy2 side connector is almost the same signal as the console ground, but it is not the exact same signal as the console ground is unfortunately not available in the GROM socket pins. This is why I recommend placing the switches between Vss on the side connector and the /BANK1 and/or /FAILSAFE pins.
Banking hardware selection
Please refer to page 5 of the presentation. This is optional.
Programming the grommy2
Please see https://github.com/Speccery/StrangeCart-Doc/blob/main/grommy2/README.md
For example GPL code, please see https://github.com/Speccery/1bpp2basic/blob/main/config.gpl . This is the source code for the grommy2 configuration utility.