A bit of history
During my time at university I became caught up in the microprocessor revolution but could not afford many of the available 8 bit microprocessor units that were available at the time, such as the MITS 8080 and the SWTP 6800. With the issue of Ron Cain's Small C compiler in an 1980 issue of Dr Dobbs I began thinking of making my own single board S100 based system. I just knew that it had to be 16 bits and it came down to a choice between the Motorola 68000 and the TI 9900. It was the uniqueness of the TMS900 architecture that set me on the path to building a couple of CPU boards around the TMS900 which eventually led to this single board computer. On this site I will share all the circuit diagrams and software that was built during the whole exercise. I believe this should be readily understandable by anyone who has experience in building electronic systems.
The TMS 9900 Microprocessor
The TMS 9900 microprocessor was released in the late 70s and was described as having a revolutionary 16 bit architecture. The original chip layout was a
fully exposed 16 bit pin out which resulted in a chip with a very large PCB 64 pin foot print. At the time they were very expensive but I was able to obtain
a couple of samples which allowed me to prototype some designs I was working on. A very early S100 version with a 64k memory board can be seen in the
photo on the left. Another S100 board for a Floppy Disc Controller (FDC) was also built. The CPU board was equipped with two EPROMs (high and low bytes) that contained the monitor based on
the BYTE Nybbles article published by BYTE in 1979. The article, "A TMS-9900 Monitor", was written by Jeremy Jones and Alan Jones of Trinity College Dublin and can be downloaded
here. and not only has a full description of a monitor for the TMS9900 family but also presents
a single stepping circuit along with memory locking.
In the early 1980s an enhanced version of the microprocessor was released. The TMS 99105 was backward-compatible with the TMS9900 but featured an on-board phase clock and multiplexed data bus that reduced the pin out and introduced a degree of additional sophistication in the manner in which the state of the microprocessor could be monitored. The first chip I managed to get access to was a TMX 99105, where the 'X' signifies that the chip was part of an experimental batch which has subsequently been replaced with a full production version. The reduced size of the chip and the integration of the clock and other improvements made it possible to include the chip and a full 64k byte memory capacity and floppy disc controller on a single wire wrap prototype board. This board is shown in the photograph on the left. Texas Instruments TMS 99105 is the third generation of 16-bit microprocessors. The family includes all instructions from the two previous generations, TMS9900 and TMS9995, and fully object-code compatible with them. New instructions included in the TMS 99105 are 32-bit arithmetic and logic, bit test, signed multiply and divide, and stack-related instructions. In addition to these new instructions, it is possible to re-define unused processor opcodes as new instructions. This can be achieved either with the help of an attached processor (i.e. co-processor), or by using a Macrostore feature. The Macrostore allows system designers to map unused microprocessor opcodes to custom functions located in a memory address space, which is separate from main memory. This allows the CPU to access up to 128 KB memory (64 KB of main memory + 64 KB of Macrostore memory). It is also possible to split main memory into two memory segments - 64 KB instruction segment and 64 KB data segment. In this case maximum addressable memory size is increased to 192 KB. The TMS99105 family uses memory-to-memory architecture. The main advantage of this architecture is that the set of CPU registers (called "Workspace") can be located anywhere in memory. This architecture makes saving and restoring of the contents of all CPU registers as simple as switching the base address of the workspace. The disadvantage of this architecture is that the processor speed is highly dependent on memory speed. The 99105 CPUs have external clock frequency 24 MHz, which is divided by four internally. It takes three or more machine cycles for the CPU to execute any instruction when no wait states are required to access the memory. This translates to maximum execution speed 2,000,000 instructions per second or less. Execution speed drops significantly when the processor is used with slow memory - instructions may execute up to two times slower when memory access requires 1 wait-state, or up to 3 times slower when 2 wait-states are required.
For further details on this microprocessor, see CPU-World.com
Description of the Single Board Computer
From the circuit diagram of the TMS99105 SBC it should be apparent that the board comprises three primary components, the CPU and its logic, the memory and its controller and lastly the FDC. The original FDC was designed to interface to an 8" floppy disc, whilst the hardware and software is present, the cirucit diagram does not show it but it can be readily added by anyone needing this. The logic on the board was built directly from the data in the TMS99105A Data manual, and the only area that may cause any concern is the logic-reducing 74S188 PROM that is used to decode some of the key internal CPU states that are used during the multiplexing of the address, data and IO states. The coding for the PROM is available here and there are a few links on the internet that show how to build a simple programmer. The ROM monitor was based on the Jeremy and Alan Jones monitor but modified to allow the loading of HEX images into memory along with some other improvements. This HEX loader is the key to bootstrapping the FDC controller software and the floppy disc bootstrap routines. The structure of the FDC Monitor was taken, in part, from the BIOS listing that was used in the Fergusson Z80 Big Board II single board computers of the early 80s.
Terminal Communications Interface
The board is designed to communicate through a standard DTE/DTA RS232 serial connection to the user terminal which will normally be a PC running a terminal application. This application can then be used to execute commands on the SBC and to download programmes, etc. A typical interface is shown on the left.
Terminal software on the PC can be readily found on the internet and you should chose the one that has the interface you feel most comfortable with. The important requirement is that the terminal programme is able to download files.
Installing the TIMON DEBUG ROMs
The DEBUG Monitor can reside in either a 2716 or 2732 (16k or 32k) ROM located at address F000H in the TMS99105's memory space and because the CPU is 16 bit requires that if you use 8bit EPROMs such as the 2716 or 2732 that you split the DEBUG Monitor HEX file into upper and lower bytes. The a99.exe assembler produces INTEL format HEX files for loading into an EPROM burner so that the EPROMs can be programmed. The complete package, including the source, HEX files and the assembler can be downloaded from here. .
The programming of the EPROMS can be a little tricky and the following memory map hopefully makes sense of it. As we are using 2732 (32k) EPROMs and because we
have our TIMON debugging monitor located at F000H we need to offset the location of code in the EPROMS 0800H. So, if you are using an EPROM burner similar to
the Wellon VP-280 (which is what I use) you would load the file odd bytes into the programme’s memory specifiying a length of 1000H and a destination location of 0800H. This would result
in an image similar to that shown in the diagram left.
The method of loading software programmes into memory and executing them is described in the Building the Small OS page.