Date   : Tue, 30 Jul 1985 17:23:57 GMT
From   : John Blalock <jb%terak.uucp@BRL.ARPA>
Subject: New Real Time Clock/Calendar Chip

Adding a Real Time Clock to a micro can be somewhat of a hassle, to say the
least.  The OKI 5832 is best interfaced thru a PIA, the National 58174 and
58274 chips have faster access times but still require wait states.  OKI,
however, has recently introduced a new RTC chip, the MSM6242RS, that is
designed to be directly interfaced to the sytem bus of Z80, 8080, 8085,
8086/8, 6800, 6502, etc. processors without the need for wait states.  It
has several neat features such as <30 microamps current draw (<10 microamps
in standby mode), +/- 30 seconds adjust, automatic leap year correction,
12/24 hour modes, and year/month/day/day of week in addition to hours/min/sec.

I have one running on my 4 MHz Z80 system now - extremely easy to interface!
The only problems were in setting the 12/24 hour mode correctly and sometimes
BUSY would never go low after once setting HOLD high.  Here are the fixes:
 To read the clock registers, you set the HOLD bit and wait for BUSY to go low.
 If it does't go low within 190 usec, reset HOLD and try again. (You'll fail on
 the first try only once per 36000 tries.)  The data sheet is not too clear on
 how to set the 12/24 hour modes.  To set 12-hour mode, output 01H, then 00H to
 Ctl Reg F.  To set 24-hour mode, output 05H, then 04H to Ctl Reg F.

If you need a RTC, check this one out. 

I have no connection with OKI Semiconductor, just a satisfied user of a good
part that I hadn't heard about until 4 days ago.  They should do a better
job of letting the world know about the 6242.

John Blalock, W7AAY

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