DS1921 datasheet(10/44 Pages) DALLAS

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DS1921H/Z

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BIT DESCRIPTION

BIT(S)

DEFINITION

TLS: Temperature Low

Alarm Search

If this bit is 1, the device will respond to a Conditional Search command

if during a mission the temperature has reached or is lower than the Low

Temperature Threshold stored at address 020Bh.

THS: Temperature High

Alarm Search

If this bit is 1, the device will respond to a Conditional Search command

if during a mission the temperature has reached or is higher than the

High Temperature Threshold stored at address 020Ch.

TAS: Timer Alarm Search

If this bit is 1, the device will respond to a Conditional Search command

if during a mission a timer alarm has occurred. Since a timer alarm

cannot be disabled, the TAF flag usually reads 1 during a mission.

Therefore it may be advisable to set the TAS bit to a 0, in most cases.

Mission Start Delay Counter

The content of the Mission Start Delay Counter determines how many minutes the device will wait before

starting the logging process. The mission start delay value is stored as unsigned 16-bit integer number at

addresses 212h (low byte) and 213h (high byte). The maximum delay is 65535 minutes, equivalent to 45

days, 12 hours, and 15 minutes.

For a typical mission, the Mission Start Delay is 0. If a mission is too long for a single DS1921H/Z to

store all temperature readings at the selected sample rate, one can use several devices, staggering the

Mission Start Delay to record the full period. In this case, the RO bit in the control register (address

020Eh) must be set to 0 to prevent overwriting of the recorded temperature log after the datalog memory

is full. See Mission Start and Logging Process description and flow chart for details.

Status Register

The Status Register holds device status information and alarm flags. The register is located at address

214h. Writing to this register will not necessarily end a mission.

Status Register Bitmap

ADDR

0214h

TCB

MEMCLR

MIP

SIP

TLF

THF

TAF

The functional assignments of the individual bits are explained in the table below. The bits MIP, TLF,

THF and TAF can only be written to 0. All other bits are read-only. Bit 3 has no function.

Status Register Details

BIT DESCRIPTION

BIT(S)

DEFINITION

TCB: Temperature Core

Busy

If this bit reads 0 the DS1921H/Z is currently performing a temperature

conversion, either self-initiated because of a mission being in progress

or initiated by a command when a mission is not in progress. The TCB

bit goes low just before a conversion starts and returns to high just after

the result is latched into the readout register at address 0211h.

MEMCLR: Memory

Cleared

If this bit reads 1, the memory pages 17 and higher (alarm time

stamps/durations, temperature histogram, excluding datalog memory),

as well as the Mission Time Stamp, Mission Samples Counter, Mission

Start Delay and Sample Rate have been cleared to 0 from executing a

Clear Memory function command. The MEMCLR bit will return to 0 as

soon as writing a non-0 value to the Sample Rate Register starts a new

mission, provided that the EM bit is also 0. The memory has to be

cleared in order for a mission to start.

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