Volume 2, Number 3 – July 1982

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by G.R.Kirby

Quelle heure est il? Ever wanted to ask your friendly Nascom that question? Well with 4 integrated circuits and a few components it is possible to build a “real-time” clock for your Nascom using the National Semiconductor chip MM 58174

The MM 58174.

The MM 58174 is a 16 pin device containing 16 internal registers programmed to give the time in tenths of a second, day of the week, day of the month and the month of the year. It does not however give the year, although, by telling it during initialization the year with respect to the last leap year, it will automatically keep the month of Febuary up to date (ie. 28 or 29 days). The circuit is controlled by its own internal oscillator, driven by an external quartz of 32768 KHz which I believe is a standard frequency for electronic time pieces (therefore, easy to obtain). The registers are addressed by a 4 bit bus (BCD). The following table gives the purpose and address of each register and also the pin connections

R0Mode Test0000(W)rite
R1Tenths of seconds0001(R)ead
R10Day of the week1010R/W
R13Year 0-3 basis1101W

The power requirement of the MM 58174 is a single 5V supply, there is however a standby mode where voltage as low as 2.2V is sufficient to keep the clock running and the time up to date. In this mode it is not possible to read or write to the internal registers. Power consumption in “stand-by” mode is less than 10uA. There are, as you will realise, many possible uses for this powerful circuit. The following is a “real-time” clock using 2 Ni-Cad batteries to keep the clock running during periods when your Nascom is switched off.

The Circuit Diagram

As you can see, relatively few components are needed to build this “real-time” clock, the principal of course being the MM 58174 from National Semiconductor which is selected by the address decoder IC 2. IC 2 is a 1-in-8 decoder being low when

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