app521| Application Note

Application Note 521 Replacement for the MC146818
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OVERVIEW
The Motorola MC146818 is no longer in production. There is no direct replacement for this part. However, systems that have been designed to use the MC146818 can be modified to operate with the Dallas Semiconductor DS12885, DS12887, DS1685, or DS1687. This application note discusses the differences between these parts and potential issues in converting from the MC146818 to the Dallas Semiconductor Real Time Clocks.

DESCRIPTION
The MC146818 provided clock and calendar functions with 50 bytes of user RAM memory, and was designed to work with either the Motorola or Intel processor timing. These features could be made nonvolatile by providing a backup battery on the board to maintain the VDD supply. For timekeeping, the MC146818 required a number of discrete components on the PCB along with a crystal to provide an input frequency. The input frequencies could be 32.768 kHz, 1.048576 MHz, or 4.194304 MHz. The DS12885 is designed to provide the same timekeeping functions with a total of 114 bytes of user NV RAM. To reduce the part count on the PCB, the DS12885 will accept direct connection of a 32.768 kHz crystal with no additional timing components required. The DS12885 also has a separate input for a backup battery with internal switching circuitry to avoid data corruption during a power fail condition. The DS12885 is available in either 24-pin DIP or 28-pin PLCC packages. The DS1685 includes an enhanced feature set including: century
byte; 64-bit serial number; wake-up alarm; kickstart; SMI recovery stack; 242 bytes of user NV RAM; and an auxiliary battery input. The timekeeping algorithm in the DS1685 includes leap year compensation valid up to 2100. The DS1685 is available in either 24-pin DIP or 28-pin PLCC packages. The DS12887 and DS1687 are modules combining a RTC with a battery and crystal contained within the package, so that no external parts are required. The DS12887 includes the DS12885 while the DS1687 is built using the DS1685. The modules has the same foot print as the 600-mil DIP package, and can therefore be placed on existing boards with a minimum of board level changes required. If replacing the MC146818 using an existing 600-mil DIP footprint is desired, then the DS12887 or DS1687 would be the preferred choice. This conversion will involve the least number of changes to the board and will work without removing the existing timing circuitry from the board. Using the DS12885 or DS1685 will require replacing the timing ci
rcuitry with a 32.768 kHz crystal attached directly to the crystal input pins. In order to make the DS12885 or DS1685 nonvolatile will also require the connection of a 3-volt battery directly to pins that have other functions on the MC146818.

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