DS1921 datasheet(15/44 Pages) DALLAS

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DS1921H/Z

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The state of the Search Condition bits in the Control Register does not affect the mission. If multiple

devices are connected to form a 1-Wire net, the setting of the search condition will enable devices to

participate in the conditional search if certain events such as timer or temperature alarm have occurred.

Details on the search conditions are found in the section ROM Function Commands later in this document

and in the Control Register description.

The setting of the RO bit (rollover enable) and sample rate depends on the duration of the mission and the

monitoring requirements. If the most recent temperature history is important, the rollover should be

enabled (RO = 1). Otherwise, one should estimate the duration of the mission in minutes and divide the

number by 2048 to calculate the value of the sample rate (number of minutes between temperature

conversions). If the estimated duration of a mission is 10 days (= 14400 minutes) for example, then the

2048-byte capacity of the datalog memory would be sufficient to store a new value every 7 minutes. If the

datalog memory of the DS1921H/Z is not large enough to store all temperature readings, one can use

several devices and set the Mission Start Delay to values that make the second device start recording as

soon as the memory of the first device is full, and so on. The RO-bit needs to be set to 0 to disable

rollover that would otherwise overwrite the recorded temperature log.

After the RO bit and the Mission Start Delay are set, the Sample Rate Register is the last element of data

that is written. The sample rate may be any value from 1 to 255, coded as an unsigned 8-bit binary

number. As soon as the sample rate is written, the DS1921H/Z will set the MIP flag and clear the

MEMCLR flag. After as many minutes as specified by the Mission Start Delay are over, the device will

wait for the next minute boundary, then wake up, copy the current time and date to the Mission Time

Stamp Register, and make the first temperature conversion of the mission. This increments both the

Mission Samples Counter and Device Samples Counter. All subsequent temperature measurements are

taken on minute boundaries specified by the value in the Sample Rate Register. One may read the

memory of the DS1921H/Z to watch the mission as it progresses. Care should be taken to avoid memory

access conflicts. See section Memory Access Conflicts for details.

MEMORY/CONTROL FUNCTION COMMANDS

The Memory/Control Function Flow Chart (Figure 10) describes the protocols necessary for accessing

the memory and the special function registers of the DS1921H/Z. An example on how to use these and

other functions to set up the DS1921H/Z for a mission is included at the end of this document, preceding

the Electrical Characteristics section. The communication between master and DS1921H/Z takes place

either at regular speed (default, OD = 0) or at Overdrive Speed (OD = 1). If not explicitly set into the

Overdrive mode, the DS1921H/Z assumes regular speed. Internal memory access during a mission has

priority over external access through the 1-Wire interface. This can affect the Read Memory commands

described below. See section Memory Access Conflicts for details.

ADDRESS REGISTERS AND TRANSFER STATUS

Because of the serial data transfer, the DS1921H/Z employs three address registers, called TA1, TA2, and

E/S (Figure 9). Registers TA1 and TA2 must be loaded with the target address to which the data will be

written or from which data will be sent to the master upon a Read command. Register E/S acts like a byte

counter and transfer status register. It is used to verify data integrity with Write commands. 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

integer 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 will begin. This address is called

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