Real Time Clock (RTC)
146818, 6242, 12887, 5C01

The part in computer that remembers the current date and time. Usually RTC chips contain some static memory with battery backup.

Below is documentation for RTCs with parallel data lines.

MC146818 and DS12887

Have 8bit data bus, multiplexed with 6 or 7bit address bus. The MC146818 needs external battery and external clock circuit that also is connected to battery. The DS12887 has battery on its back (however, you cannot exchange it to a new one).

Were used in PC AT computers (now integrated; for XT was other RTC). Currently used in Sam Coupé ("Dallas clock") and YAMOD.ATBUS. Not recommended in new designs, as are big and make trouble when used with Z80CPU (due to decoding).

MSM6242B and RTC62421/RTC62423

Have 4bit data bus and separate 4bit address bus. Need external battery. The MSM6242B needs also external clock, while the RTC62421 (DIL18) and RTC62423 (SMD SO-24) have internal oscillator.

Were used in Amiga and Sam Coupé (Sambus). Now used in Atari 8bit.

RP5C01/RF5C01

The pinout is similar to 6264 family clocks (4bit data, 4bit address), but incompatibile. Have four sets of registers: the second are alarm settings and next two are memory cells (104 bits).

Used in Amiga (in place of 6242) and MSX-2.

M48Z08 oraz M48Z18

It is very interesting solution - 8kB SRAM with some cells that behaves like RTC. Unfortunately is 10 times expensiver than SRAM of that size.

Files for download

rtcx-001.zip 26.1kB (2006-04-03 21:30)

Small board that converts RTC72421 (DIL) into MC146818 socket. Eagle and Tango PCB data files.

rtc.zip 2.5kB (2005-05-02 15:35)

Real Time Clock -- Eagle library "rtc.lbr".

rtc62423.pdf 126.4kB (2005-04-28 23:55)

RTC62423 by EPSON.

rtc7242x.pdf 66.0kB (2005-04-13 01:02)

RTC72421 (DIL) and RTC72423 (SMD) datasheet.

mc146818.pdf 2121.8kB (2004-11-10 01:20)

Archive datasheet for MC146818 from Motorola, 1988.

12887.pdf 584.9kB (2003-06-12 18:54)

DS12887 datasheet from Dallas.

msm6242b.pdf 177.9kB (2002-10-12 03:05)

MSM6242B by OKI.

ds12887.pdf 349.2kB (2002-10-11 15:00)

DS12887 datasheet from Maxim.

m48z08.pdf 114.5kB (1999-06-03 11:05)

M48Z08 and M48Z18 - 8kB SRAM with RTC.

rf5c01a.pdf 124.4kB (1998-11-19 18:23)

RP/RF 5C01 A by Ricoh datasheet.

Links to other pages

See also

IIC Real Time Clock (RTC) PCF8583 DS1307

IIC Real Time Clock (RTC). Serial data transfer. PCF8583, DS1307.