Copyright 2017 Eric Smith spacewar@gmail.com
elf-clock development is hosted at the elf-clock Github repository.
This program use a COSMAC Elf microcomputer (CDP1802 microprocessor) with a PIXIE graphics chip (CDP1861) to display a digital clock on an NTSC monitor. Only 256 bytes of RAM are required, as provided on an unexpanded Elf or Elf II, though the program will also work if more RAM is present. By default a 12-hour clock is provided, but the line "clkhrs equ 12" can be changed to "clkhrs equ 24" to assemble a 24-hour clock instead.
Pressing and holding the Elf INPUT button will activate the "fast" set mode, which counts at 60 times normal speed (one minute per second), in order to allow setting the current time. Unfortunately that's still quite slow, so I recommend setting the clock not too long after midnight, or for the 12-hour clock, after noon.
As provided, the timing assumes that the Elf clock is provided by a 3.579545 MHz crystal (NTSC color burst) divided by two, which is a common configuration for an Elf with a CDP1861 PIXIE graphics chip, resulting in a field rate of 60.9928 Hz. The program divides this by 61 to get seconds, so the error is fairly small. A trimmer capacitor could be added to the crystal oscillator circuit to allow fine adjustment.
If you have an Elf with a 1.76064 MHz clock, the field rate is 60.0000 Hz, so you can change the byte at address 00B5 in the program from 3D to 3C to divide by 60 rather than 61.
If you have an Elf with some other clock frequency, there will be greater error in the timekeeping. C'est la vie.
The program occupies 254 bytes out of the 256 bytes of memory of an unexpanded Elf, with the remaining 2 bytes used for the stack.
This has been tested on a Netronics ELF II, and on the EMMA 02 emulator.
This program is free software: you can redistribute it and/or modify it under the terms of version 3 of the GNU General Public License as published by the Free Software Foundation.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.