Rainbow Electronics DS1990A - Manual (Page 2)

DS1990A
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iButton DESCRIPTION
The DS1990A Serial Number iButton is a rugged data carrier that acts as an electronic registration
number for automatic identification. The DS1990A consists of a factory-lasered, 64-bit ROM that
includes an unique 48-bit serial number, an 8-bit CRC and an 8-bit Family Code (01h). Data is transferred
serially via the 1-Wire protocol which requires only a single data lead and a ground return. The DS1990A
is fully compatible with the DS1990 Serial Number iButton but provides the additional 1-Wire protocol
capability that allows the Search ROM command to be interpreted by the DS1990A and therefore allows
multiple DS1990A devices to reside on a single data line.
The durable MicroCan package is highly resistant to environmental hazards such as dirt, moisture and
shock. Its compact coin-shaped profile is self-aligning with mating receptacles, allowing the DS1990A to
be used easily by human operators. Accessories permit the DS1990A to be mounted on plastic key tabs,
photo ID badges, printed circuit boards or any smooth surface of an object. Applications include access
control, work-in-progress tracking, tool management and inventory control.
OPERATION
The DS1990A’s internal ROM is accessed via a single data line. The 48-bit serial number, 8-bit family
code and 8-bit CRC are retrieved using the Dallas 1-Wire protocol. This protocol defines bus transactions
in terms of the bus state during specified time slots that are initiated on the falling edge of sync pulses
from the bus master. All data is read and written least significant bit first.
1-WIRE BUS SYSTEM
The 1-Wire bus is a system which has a single bus master system and one or more slaves. In all instances,
the DS1990A is a slave device. The bus master is typically a microcontroller. The discussion of this bus
system is broken down into three topics: hardware configuration, transaction sequence, and 1-Wire
signaling (signal type and timing). For a more detailed protocol description, refer to Chapter 4 of the
Book of DS19xx iButton Standards.
Hardware Configuration
The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to
drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have an
open drain connection or 3-state outputs. The DS1990A is an open drain part with an internal circuit
equivalent to that shown in Figure 2. The bus master can be the same equivalent circuit. If a bidirectional
pin is not available, separate output and input pins can be tied together. The bus master requires a pullup
resistor at the master end of the bus, with the bus master circuit equivalent to the one shown in Figure 3.
The value of the pullup resistor should be approximately 5 k
Ω
for short line lengths. A multidrop bus
consists of a 1-Wire bus with multiple slaves attached. The 1-Wire bus has a maximum data rate of 16.3k
bits per second.
The idle state for the 1-Wire bus is high. If, for any reason, a transaction needs to be suspended, the bus
MUST be left in the idle state if the transaction is to resume. If this does not occur and the bus is left low
for more than 120
µ
s, one or more of the devices on the bus may be reset.