Nascom News­let­ter

perhaps day of week. I intend having only ong routine to read the clock, which mill form part of a sat of extended monitor Any program requiring clock data will call this and ‘from its information select the relevant data. chip changes (ticks?) between twa reads, the register In this case, Recessary to reread the Completely. – National Semiconductor recommend that you should wait for a 0FH signal from the clock before embarking on a full read. The routines listed here print “FAIL” whenever a read fails for this reason, and proceed to try again. The decision on how best to cope with this T leave to you’ Setting up a version of the same routines to read the clock and print the time on the screen indicates that a read takes approx 1 nS.

In using this chip, a useful adjunct is to acquire a copy of the National Semiconductor data sheet, which gives an overall view of the intricacies of the chip, in particular use of the interrupt. I’ve not achieved 100% reliability in ay use Of the interrupts and have therefore disregarded them in’ the interests of getting on with things, but I feel sure that this circuit will allow them to be used reliably. The softnare is probably capable of considerable improvement, but as listed here it works on a 4 Mhz machine. 2 Mhz may cause problems, as the width of the NRDG (Read strobe) should be limited to) 15u5 max according to the datasheet.

Tam very happy with this chip and recommend it without reserve in the circuit attached. If you are into Clocks, the May ‘82 BYTE, in Ciarcia’s Circuit Cellar, shows a circuit’ for another NS ‘chip. This chip is the MMSG1G7A, which is slightly more expensive than the SB174, but offers a’ number of extra Te will fom 1/10000 secu up to months, but knows nothing about leapyears. In addition, it has a low ‘power interrupt ‘allowing it when powered down to signal an interrupt when a specified tine and date is reached. This could offer the possibility of your machine being able to turn itself fon on Tuesdays at 2.50 p.m. to da whatever you wish! I intend to build a number of special

seriously considering incorporating the 58167A into then.

s TITLE*CLOCK DRIVER * 2 CRT: EQU 65H

3 CRLF: EDU GAH

4 MRET: EGU SBH

5 B2HEX: EDU 6eH

& CLEREG: EQU OSH Clk regs lines on port B 7 SPACE: EDU 49H

8 CLKDAT: EQU O4H Clk data Lines on port A 9

CR: | EGU ODH 10 BITHOD: EQU OFFH ;Token to set PIO

an INMSK: EGU 1FH;Bottom five lines are input 12 QUTMGK: EQU 10H yInterrupt line im output