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REPLACING 8051 RAM

WITH DALLAS TIMEKEEPING RAM
by Bruce Renyolds

Start ý Clock Operation ý Donýt Need 32 KB? ý Replacing EPROMs ý Sources and PDF

CLOCK OPERATION

The clock registers reside at the top eight RAM locations 7FF8ý7FFF and are accessed using the control register, as shown in Figure 1. Most designers will appreciate this approach because it requires no programming wizardry or additional circuitry to gain access to date and time information.

Date and time information is stored in BCD format for year, month, date, day, hours, minutes, and seconds. The internal clock register structure is double-buffered to prevent reading incorrect data during clock update cycles.

Setting bit 6 at location 7FF8 to a logic 1 (the Read bit) halts updates to the internal clock registers and enables the program to read the date and time information. Clock accuracy is guaranteed to be within ý1 min./mo. at 25ý C.

DATE/TIME REGISTERS

Accessing the internal clock registers is simple and straightforward. Listing 1 is a sample of 8051 assembly code to demonstrate clock-access methods.

To begin using the clock functions, call subroutine START_OSC to fire up the internal oscillator and start the clock ticking.

Calling subroutine STOP_OSC can be used to increase the shelf life and minimize current drain on the internal lithium battery. However, stopping the internal oscillator cancels clock register updates. This option is useful if you need to remove the device from the application circuit for storage.

POWERCAP MODULE BOARD

For applications requiring a low-profile version of the DS1644, Dallas offers the DS1644P powercap module board that mates with the DS9034PCX (powercap). The DS1644P is a surface-mount package that connects directly to the powercap containing the battery and crystal, as illustrated in Figure 3.

Figure 3ýFor stackable controllers, or just to save space on a new design the DS1644P 34-pin low profile powercap module board offers an alternate design solution. This package fits into a standard 68-pin PLCC surface mountable socket and is functionally equivalent to the DS1644 DIP.

The powercap module in Figure 4 mounts on top of the DS1644P after the ý1644P is soldered in place. This setup prevents damage to the crystal and battery inside the DS9034PCX from the high temperatures during the solder reflow process of the ý1644P. The powercap is also keyed to prevent it from being attached to the top of the ý1644P in reverse order.

Figure 4ýThe DS9034PCX powercap modules contain the lithium energy cell and clock crystal that simply snap over the top of the DS1644P.

Although the DS1644P offers a reduced profile, with a guaranteed clock accuracy of ý1.53 min./mo. at 25ý C, its accuracy is slightly lower than the DS1644 DIP.

With this setup, replacing the battery would only be a matter of popping off the ý9034PCX. Of course, manufacturers can ship the ý9034PCX module unattached to preserve battery life until installation in the target system.

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