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BUILDING AN EMBEDDED TIMING SYSTEM

by Jamie Pollock

Start ı Problem Solving ı EEPROM Programming ı Some Assemblyı ı Interfacing the LCD ı Sources and PDF

PROBLEM SOLVING

A couple of obstacles are created with a simple program like this, but if the RAM can be mine, then so can the world! First, the bootstrap fills the RAM from the address $0000 up. This seems like a good idea until one realizes that the vector table for bootstrap mode points to the block $00C4 to $00FD, right in the beginning of the 1-KB on-chip RAM ($0000 to $03E8).

Second, the EEPROM is only 512 bytes. Because I plan to store my program in EEPROM and have the controller execute my code at power-up, I need to have a block of 512 bytes of RAM available in one chunk. I also need to use interrupts so a patch will be created.

The first attempt to resolve the interrupt address conflict was to use a free program from Motorola called PCBUG11. This is a great tool for working with the ı6811. If the controller doesnıt have external RAM, then the software uses up all available RAM.

My solution is to write a small bootloader that imitates the original bootstrap code but places the program at $0100 then times out and jumps to $0100 to execute my code (see Listing 1).

The 6811F1 has 1 KB of RAM. If I start my code at $0100, there are 744 bytes available in one block. This gives me 512 bytes with 256 bytes for the stack. There is unused RAM below $00C4 that I will use for variables. Now Iım ready to download and execute my assembled ıS19 file from RAM. A simple BASIC program will communicate with the controller. The terminal program must do several different tasks. First, it must establish communications and send the first piece of codeıthe bootloader.

The controller accepts the code in binary sequential format. The ıS19 file must first be translated to remove the header, address, and the checksum. I have modified the Motorola application note AN 1260 for making this change to the bootloader and to the project ıS19 files.

Bootload.bas is the final PC terminal program. It sends the bootloader code then prompts the user for the name of the ıS19 file to be sent. Bootload.bas processes the ıS19 files and places the raw code into an array. The array is then dumped to the controller, echoing the response to the PC screen for verification. When the code finishes sending the Bootload.bas program, it becomes a terminal emulator for debugging.

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