The French Connection
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
| Register | AD3 | AD2 | AD1 | AD0 | MODE |
| R0 | Mode Test | 0 | 0 | 0 | 0 | (W)rite |
| R1 | Tenths of seconds | 0 | 0 | 0 | 1 | (R)ead |
| R2 | Seconds | (units) | 0 | 0 | 1 | 0 | R |
| R3 | Seconds | (tens) | 0 | 0 | 1 | 1 | R |
| R4 | Minutes | (units) | 0 | 1 | 0 | 0 | R/W |
| RS | Minutes | (tens) | 0 | 1 | 0 | 1 | R/W |
| R6 | Hours | (units) | 0 | 1 | 1 | 0 | R/W |
| R7 | Hours | (tens) | 0 | 1 | 1 | 1 | R/W |
| R8 | Days | (units) | 1 | 0 | 0 | 0 | R/W |
| R9 | Days | (tens) | 1 | 0 | 0 | 1 | R/W |
| R10 | Day of the week | 1 | 0 | 1 | 0 | R/W |
| R11 | Month | (units) | 1 | 0 | 1 | 1 | R/W |
| R12 | Month | (tens) | 1 | 1 | 0 | 0 | R/W |
| R13 | Year 0-3 basis | 1 | 1 | 0 | 1 | W |
| R14 | Stop/Go | 1 | 1 | 1 | 0 | W |
| R15 | Interrupt | 1 | 1 | 1 | 1 | R/W |
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
