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		So what more is there to add? Plenty I think, but first of all, some may wish to know just how much power the clock consumes when considering the power supply. The picture below shows a consumption of nearly 165mA with a 5.02V supply rail, with standard Z80A, standard 2764 and standard 74LS logic components. Note that by substituting the Z80 and the 2764 for CMOS components, and HC logic for the LS TTL, the consumtion will drop considerably. Remember for an even lower figure when using CMOS, keep the system clock frequency down!


	USER OPTIONS

Whilst our project clock now appears to be working, we still haven�t given any thought to the interface devices such as the display, the keypad, or the reference timebase. OK so lets start off with the KEYPAD. The basic Digital Clock uses a total of 5 push switches (and a reset button if you wish). The inputs to these should be made directly to the 74xx374 pins, with 4K7 pull-ups and 0.1uF caps across the contacts. (The occasional bit of key-bounce can probably be tolerated as the keys are are only used for initial seting up, so the caps can be left off if preferred.)

I used the DL1414 display for pure convenience to demonstrate this circuit. They are a bit pricey when new, but can often be found in surplus equipment. If they are NOT used, then alternative arrangements will have to be made to supply 4 data LATCHES, a simple ADDRESS DECODER (from the top 4 bits of each display data-word) and some form of 7 segment DISPLAY DECODER.


	The TIMEBASE is the frequency souce used by the clock for accurate timekeeping. Whilst it would have been quite acceptable to divide down the IMHz system clock to 50Hz in the case of this project, for others using more obscure frequencies, this may prove to be a nigh impracticable feat. Thus we must consider a viable alternative. In the case of UK residents and other countries using a 50Hz mains frequency, we can use this as the basis for our timebase by using a circuit such as the one on



	Programmer


	the left. This uses an Opto coupler to interface the raw output from a mains PSU transformer to give us our 50Hz. Of course US citizens can use the same principle by altering the frequency divide value in the program from 51 to 61 (for 60Hz) This means altering the value in �Digital Clock� Rev 1.0 program location 00ACh from 33h, to 3Dh. I just happened to have an old used 6N137 to hand, so I used this. Almost any sort may be used for this application but do check the pinout first!



	VIDEO info


		Peripheral circuitry


Z280 and Central Heating Controllers


			Concluding ideas

FURTHER USEFUL INFORMATION

Whilst the RAM is not used for the basic clock program, the addressing is set up to access the RAM from address 4000h. The circuit supplied here is wired for a 6116 2Kx8 (or equivalent) static RAM. If using more advanced compatible programs such has �Stopwatch 3, then the RAM will need to be fitted.

If anyone building the basic project wishes to add a �seconds� LED (to essentially show that the clock is running), a �seconds pulse� is available by decoding a 4xh on the outport lines to the display. If a separate display decoder such as the 74xx138 is already being used (in place of the DL1414 I used here), then this output may well be already available on one of the pins. Bear in mind that a PULSE STRETCHER circuit (in the form of a one-shot) will be needed to lengthen the pulse - which will otherwise be too short to be clearly seen.

DONT FORGET to tie any unused input pins to a logic high, particularly if they are CMOS inputs.


		Links

	http://www.hampshire-shops.co.uk


	DIGITAL CLOCK PROGRAM LISTING


	DIGITAL CLOCK WIRING NETLIST


	BASIC DIGITAL CLOCK SCHEMATIC


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