DS1994

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Figure 3. 1-WIRE CRC CODE

X

0

X

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Polynomial = X

1

st

STAGE

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nd

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rd

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th

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th

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th

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th

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th

STAGE

INPUT DATA

MEMORY

The memory map in Figure 4 shows a 32-Byte page called the scratchpad, and additional 32-Byte pages

called memory. The DS1994 contains 16 pages that make up the 4096-bit SRAM. The DS1994 also

contains page 16, which has only 30 Bytes containing the timekeeping registers.

The scratchpad is an additional page that acts as a buffer when writing to memory. Data is first written to

the scratchpad where it can be read back. After the data has been verified, a copy scratchpad command

transfers the data to memory. This process ensures data integrity when modifying the memory.

TIMEKEEPING

A 32.768kHz crystal oscillator is used as the time base for the timekeeping functions. The oscillator can

be turned on or off by an enable bit in the control register. The oscillator must be on for the real-time

clock, interval timer, and cycle counter to function.

The timekeeping functions are double buffered. This feature allows the master to read time or count

without the data changing while it is being read. To accomplish this, a snapshot of the counter data is

transferred to holding registers that the user accesses. This occurs after the 8th bit of the read memory

function command.

Real-Time Clock

The real-time clock is a 5-Byte binary counter. It is incremented 256 times per second. The least

significant Byte is a count of fractional seconds. The upper 4 Bytes are a count of seconds. The real-time

clock can accumulate 136 years of seconds before rolling over. Time/date is represented by the number of

seconds since a reference point, which is determined by the user. For example, 12:00 A.M., January 1,

1970 could be a reference point.