DS1821

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READ/WRITE TIME SLOTS

DS1821 data is read and written through the use of time slots to manipulate bits and a command word to

specify the transaction.

Write Time Slots

A write time slot is initiated when the host pulls the data line from a high logic level to a low logic level.

There are two types of write time slots: Write 1 time slots and Write 0 time slots. All write time slots

must be a minimum of 60

µs in duration with a minimum of a 1-µs recovery time between individual

write cycles.

The DS1821 samples the DQ line in a window of 15

µs to 60 µs after the DQ line falls. If the line is high,

a Write 1 occurs. If the line is low, a Write 0 occurs (see Figure 6).

For the host to generate a Write 1 time slot, the data line must be pulled to a logic low level and then

released, allowing the data line to pull up to a high level within 15 microseconds after the start of the

write time slot.

For the host to generate a Write 0 time slot, the data line must be pulled to a logic low level and remain

low for the duration of the write time slot.

Read Time Slots

The host generates read time slots when data is to be read from the DS1821. A read time slot is initiated

when the host pulls the data line from a logic high level to logic low level. The data line must remain at a

low logic level for a minimum of 1 µs; output data from the DS1821 is then valid for the next 14 µs

maximum. The host therefore must stop driving the DQ pin low in order to read its state 15 µs from the

start of the read slot (see Figure 6). By the end of the read time slot, the DQ pin will pull back high via the

external pullup resistor. All read time slots must be a minimum of 60 µs in duration with a minimum of a

one µs recovery time between individual read slots.

master sample time towards the end of the 15-µs period.

INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 5