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give rise to the need for the transistor regulator drawn by Mr. Kirby. It is out of specification to connect voltage levels to inputs of CMOS chips which are more than 0.5 volts above the power supply levels of the chip. Doing this can cause the chip to latch up, with consequent data loss. That is why the need for the transistor circuit arises. A simpler method is shown in my circuit. The Diode D1 can be a Germanium OA45 or similar, which can offer a Vf(forward voltage drop) of approx 0.5 volt. Unfortunately, such a diode will have an Ir(reverse leakage) of typically 50uA. This leakage current is some five to ten times the power down supply current of the 58174. In consequence, the battery only lasts i/6th as long as it should (approx three weeks opposed to 4-6 months). Replacing the Germanium diode with a 1N4148 or similar may give the latch up problem, as Vf over such a diode is usually 0.7volts. The reverse leakage is only 0.025uA, so we don’t affect our power consumption dramatically. Electrovalue list a Silicon Schottky Barrier Diode BAS 70-03 with Vf = 0,410 volts and Ir < 200 nA (0.0002uA). This diode costs a matter of pence, and cures the problem of latch up, providing a very simple circuit.

It is possible to obtain a very neat Nicad battery, listed in several suppliers catalogues as PCB Battery, which makes a very compact power supply for battery back up. The whole unit can be made on a 2″ square Veroboard, and works most reliably. The variable capacitor is in theory capable of adjusting the timekeeping of the clock to allow for variations in crystal and temperature, but can be omitted, if you don’t mind resetting the clock every few months to adjust the seconds. I must admit that inspite of having the variable capacitor, I’ve not been able to adjust my clock to keep pace with the pips on the BBC. It runs either two secs a day fast or slow, and I can’t get any adjustment between those limits. If rebuilding the unit, I’d be very tempted to omit the variable capacitor in the interests of constructional simplification. One very important point – the power supplies for the 4049 go to pin 1 (+5v)[pin one] and pin 8 (0v)!

The software consists of setting up address and control lines on one port, reading the data in on the other, and resetting the control lines. As I list it, the software is rudimentary and extremely inelegant, but it will serve to show that your board works. It sets the clock with the time stored in memory at 0D80H, and using the routine at 0D00H causes the clock to be read and the time printed on the screen. This routine is no practical use, as it prints the information obtained from the clock as a string of figures. If you E0D00 twice in succession, you will see the time change on the righthand side of the strings. I leave the organisation of your own reading routines to you. In assembler listings, I print date and time at the head of each page. The time is relevant there, as one may have a few printouts in the course of an evening, and the time serves to distinguish between then. On the other hand, my word processor requires only the date and