DS1990 DALLAS [Dallas Semiconductor], DS1990 Datasheet - Page 6

• Manufacturer Part Number: DS1990
• Description: Serial Number iButton
• Manufacturer: DALLAS [Dallas Semiconductor]

1-WIRE SIGNALING
The DS1990A requires strict protocols to ensure data integrity. The protocol consists of four types of
signaling on one line: Reset sequence with Reset Pulse and Presence Pulse, write 0, write 1 and read data.
All these signals except presence pulse are initiated by the bus master.
The initialization sequence required to begin any communication with the DS1990A is shown in Figure 5.
A Reset Pulse followed by a Presence Pulse indicates the DS1990A is ready to send or receive data given
the correct ROM command.
The bus master transmits (T
releases the line and goes into receive mode (R
5 k pullup resistor. After detecting the rising edge on the data contact, the DS1990A waits (t
READ/WRITE TIME SLOTS
The definitions of write and read time slots are illustrated in Figure 6. All time slots are initiated by the
master driving the data line low. The falling edge of the data line synchronizes the DS1990A to the
master by triggering a delay circuit in the DS1990A. During write time slots, the delay circuit determines
when the DS1990A will sample the data line. For a read data time slot, if a “0” is to be transmitted, the
delay circuit determines how long the DS1990A will hold the data line low overriding the 1 generated by
the master. If the data bit is a “1”, the iButton will leave the read data time slot unchanged.
INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 5
s) and then transmits the presence pulse (t
In order not to mask interrupt signaling by other devices on the 1-Wire bus, t
be less than 960 s.
X
) a reset pulse ( a low signal for a minimum of 480 s). The bus master then
RESISTOR
MASTER
DS1990A
PDL
, 60-240 s).
6 of 10
X
). The 1-Wire bus is pulled to a high state via the
480 s
480 s
15 s
60 s
t
t
PDH
PDL
t
t
RSTL
RSTH
< 240 s
< 60 s
<
<
*
(includes recovery time)
RSTL
+ t
R
should always
PDH
DS1990A
, 15-60