DS1925 datasheet(29/46 Pages) MAXIM

29 / 46 page

and the parameter is 01h, the device clears the mission log

memory, mission timestamp, mission samples counter, and

all alarm flags of the Alarm Status register. After these cells

are cleared, the MEMCLR bit of the General Status register

reads 1 to indicate the successful execution of the XPC

Clear Memory command. This command and parameter

clears the RTC (E0SC = 0). The Mission Backup registers

are written on a XPC Clear Memory (log) command. See

Figure 15.

If the parameter is 00h, only the 16 pages of user memory

are cleared. The RTC need not be enabled to clear the

user memory. The user memory can be cleared during a

mission.

XPC Forced Conversion [4Bh]

The XPC forced conversion command can be used to

measure the temperature without starting a mission. The

length for this XPC command is one byte which is the

command code. After the command code, the master

has to send one FFh byte to get the conversion started.

The conversion result is found as a 16-bit value in the

Latest Temperature Conversion Result register. This com-

mand is only executed if no mission is in progress (MIP

= 0). It takes maximum tLSTD to complete or tLSTD +

tSRTC if the device is in the Clear Memory state or in an

unknown state. After the delay, an additional FFh byte is

read followed by the temperature conversion result and a

CRC16. Additional measurements can then be taken by

issuing the first FFh byte again to repeat the sequence.

The Device Samples Counter does not increment for the

XPC Forced Conversion command.

XPC Read Battery Voltage [33h]

The XPC read battery voltage command can be used to

measure the current battery voltage. The length for this

XPC command is one byte which is the command code.

After the command code the master has to send one FFh

byte to get the voltage conversion started. This command

is only executed if no mission is in progress (MIP = 0). It

cannot be interrupted and takes maximum tLSTD to com-

plete or tLSTD + tSRTC if the device is in the Clear Memory

state or an unknown state. During this time, memory

access through the 1-Wire interface is blocked. The

device behaves the same way as during a mission when

the sampling interferes with a memory/control function

command. After the delay, an additional FFh byte is read

followed by the voltage conversion result and a CRC16.

The 16-bit integer results can be converted to a voltage

by dividing by 1024. Additional measurements can then

be taken by issuing the first FFh byte again to repeat the

sequence. This function is not available when the device

is in the start state from factory until an XPC clear memory

(log) sequence is done. At room temperature, a battery

voltage of 2.5V or lower is considered marginal.

Memory Access Conflicts

While a mission is in progress or while the device is

waiting for a temperature alarm to start a mission, peri-

odically a temperature sample is taken and logged. This

internal activity has priority over 1-Wire communica-

tion. Consequently, device-specific commands (excluding

ROM function commands, 1-Wire reset, and read/write

scratchpad for the DS1925) do not perform properly when

internal and external activities interfere with each other.

Not affected are the commands start mission, forced

conversion, and XPC clear memory (log), because they

are not applicable while a mission is in progress or while

the device is waiting for a temperature alarm. Table 26

explains how the remaining three commands are affected

by internal activity, how to detect this interference, and

how to work around it.

When writing driver software, it is important to know about

the possibility of interference and to take measures to

work around it.

Mission Backup Registers

The Mission Backup registers are used to provide a

recovery mechanism for a mission that stops due to a

depleted battery. The XPC Read Memory command func-

tions in a battery depleted condition. This condition is

detected if the backup RTC is all FFs, the backup Mission

Time Stamp is all FFs, the MIP bit is not set, and Mission

Samples Count is zero. The Mission Samples Count can

then be recovered by reading the log data until FFs are

detected. The Mission Time Stamp can be recovered from

the Mission Backup registers. See Figure 15 to see what

registers are written to the Mission Backup registers.

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

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