DS1925 datasheet(19/46 Pages) MAXIM

19 / 46 page

If the Start Upon Temperature Alarm mode is chosen

(SUTA = 1) and temperature logging is enabled (ETL =

1), the DS1925 first waits until the start delay is over.

Then the device wakes up in intervals as specified by

the sample rate and EHSS bit and measure the tempera-

ture. This increments the Device Samples Counter and

Mission Samples Counter. The first sample of the mission

is logged when the temperature alarm occurred. All sub-

sequent log entries are made as specified by the value in

the Sample Rate register and the EHSS bit.

The general-purpose memory operates independently

of the other memory sections and is not write protected

during a mission. All the DS1925’s memory can be read

at any time, i.e., to watch the progress of a mission.

Attempts to read the passwords read FFh bytes instead of

the data that is stored in the password registers.

Memory Access

Address Registers and Transfer Status

Because of the serial data transfer, the DS1925 employs

three address registers: TA1, TA2, and E/S (Figure 8).

Registers TA1 and TA2 must be loaded with the target

address to which the data are written or from which

data are sent to the master upon a Read command.

mands. Therefore, the master only has read access to

this register. The lower 5 bits of the E/S register indicate

the address of the last byte that has been written to the

scratchpad. This address is called Ending Offset. Bit 5 of

the E/S register, called PF or “partial byte flag,” is set if

the number of data bits sent by the master is not an inte-

ger multiple of 8. Bit 6 is always a 0. Note that the lowest

5 bits of the target address also determine the address

within the scratchpad, where intermediate storage of

data begins. This address is called byte offset. If the tar-

get address for a write command is 13Ch, for example,

then the scratchpad stores incoming data beginning at

the byte offset 1Ch and is full after only 4 bytes. The

corresponding ending offset in this example is 1Fh. For

best economy of speed and efficiency, the target address

for writing should point to the beginning of a page, i.e.,

the byte offset is 0. Thus, the full 32-byte capacity of the

scratchpad is available, resulting also in the ending offset

of 1Fh. The ending offset together with the PF is mainly a

means to support the master checking the data integrity

after a write command. The highest valued bit of the E/S

that a valid copy command for the scratchpad has been

received and executed. Writing data to the scratchpad

clears this flag.

Writing with Verification

To write data to the DS1925, the scratchpad has to be

used as intermediate storage. First, the master issues

the Write Scratchpad command to specify the desired

target address, followed by the data to be written to the

scratchpad. In the next step, the master sends the Read

Scratchpad command to read the scratchpad and to

verify data integrity. As preamble to the scratchpad data,

the DS1925 sends the requested target address TA1 and

TA2 and the contents of the E/S Register. If the PF flag

is set, data did not arrive correctly in the scratchpad. The

Figure 8. Address Registers

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 19

BIT

TARGET ADDRESS (TA1)

TARGET ADDRESS (TA2)

T15

T14

T13

T12

T11

T10

ENDING ADDRESS WITH

DATA STATUS (E/S)

(READ ONLY)

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