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MAX6901

3-Wire Serial RTC in a TDFN

Maxim Integrated

MAX6901 Datasheet PDF : 18 Pages

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3-Wire Serial RTC in a TDFN

Table 3. HEX Register Address/Description (continued)

WRITE

ADDRESS/COMMAND

BYTE (HEX)

READ

ADDRESS/COMMAND

BYTE (HEX)

DESCRIPTION

RAM 18

RAM 19

RAM 20

RAM 21

RAM 22

RAM 23

RAM 24

RAM 25

RAM 26

RAM 27

RAM 28

RAM 29

RAM 30

RAM Burst

POR CONTENTS

(HEX)

Indeterminate

Nonapplicable

does not happen coincident with a Seconds counter

increment to ensure accurate time data is being read.

The clock counters continue to count and keep accu-

rate time during the Read operation.

The simplest way to read the timekeeping registers is to

use a Burst Read. In a Burst Read, the main timekeep-

ing registers (Seconds, Minutes, Hours, Date, Month,

Day, Year) and the Control register are read sequential-

ly in the order listed with the Seconds register first.

They are read out as a group of eight registers, with 8

bits each. All timekeeping registers (except Century)

are latched upon the receipt of the Burst Read com-

mand. The worst-case error between the “actual” time

and the “read” time is 1 second for a normal data transfer.

The timekeeping registers may also be read using

Single Reads. If Single Reads are used, it is necessary

to do some error checking on the receiving end,

because it is possible that the clock counters could

change during the Read operations, and report inaccu-

rate time data. The potential for error is when the

Seconds register increments before all the registers are

read. For example, suppose a carry of 13:59:59 to

14:00:00 occurs during Single Read operations. The

net data read could be 14:59:59, which is erroneous.

To prevent errors from occurring with Single Read oper-

ations, read the Seconds register first (initial-seconds)

and store this value for future comparison. After the

remaining timekeeping registers have been read,

reread the Seconds register (final-seconds). Check that

the final-seconds value equals the initial-seconds value;

if not, repeat the entire Single Read process. Using

Single Reads at a 100kHz serial speed, it takes under

2.5ms to read all seven of the timekeeping registers,

including two reads of the Seconds register.

Example: Reading the Clock

with a Burst Read

To read the time with a Burst Read, send BFh as the

Address/Command byte. Then clock out 8 bytes,

Seconds, Minutes, Hours, Date of the month, Month,

Day of the week, Year, and finally the Control byte. All

data are output LSB first. Decode the required informa-

tion based on the register definitions listed in Table 2.

Using the Alarm

A polled alarm function is available by reading the ALM

OUT bit. The ALM OUT bit is D7 of the Minutes time-

keeping register. A logic 1 in ALM OUT indicates the

alarm function is triggered. There are eight registers

associated with the alarm function, seven programma-

ble Alarm Threshold registers and one programmable

Alarm Configuration register. The Alarm Configuration

are compared to the timekeeping registers, and the

ALM OUT bit sets if the compared registers are equal.

Table 2 shows the function of each bit of the Alarm

Configuration register. Placing a logic 1 in any given bit

of the Alarm Configuration register enables the respec-

10 ______________________________________________________________________________________

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