DS1985 Maxim, DS1985 Datasheet - Page 19

• Manufacturer Part Number: DS1985
• Description: The DS1985 16Kb Add-Only iButton is a rugged read/write data carrier that identifies and stores relevant information about the product or person to which it is attached
• Manufacturer: Maxim

DS1985
Match ROM [55H]
The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS1985 on a multidrop bus. Only the DS1985 that exactly matches the 64-bit ROM sequence
will respond to the subsequent memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a Reset Pulse. This command can be used with a single or multiple devices on the
bus.
Skip ROM [CCH]
This command can save time in a single drop bus system by allowing the bus master to access the
memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus
and a read command is issued following the Skip ROM command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open drain pulldowns will produce a wired-AND result).
Search ROM [F0H]
When a system is initially brought up, the bus master might not know the number of devices on the 1-
Wire bus or their 64-bit ROM codes. The Search ROM command allows the bus master to use a process
of elimination to identify the 64-bit ROM codes of all slave devices on the bus. The ROM search process
is the repetition of a simple three-step routine: read a bit, read the complement of the bit, then write the
desired value of that bit. The bus master performs this simple, three-step routine on each bit of the ROM.
After one complete pass, the bus master knows the contents of the ROM in one device. The remaining
number of devices and their ROM codes may be identified by additional passes. See Chapter 5 of the
Book of DS19xx iButton Standards for a comprehensive discussion of a ROM search, including an actual
example.
1-Wire Signaling
The DS1985 requires strict protocols to ensure data integrity. The protocol consists of five types of
signaling on one line: Reset Sequence with Reset Pulse and Presence Pulse, Write 0, Write 1, Read Data
and Program Pulse. All these signals except Presence Pulse are initiated by the bus master. The
initialization sequence required to begin any communication with the DS1985 is shown in Figure 9. A
Reset Pulse followed by a Presence Pulse indicates the DS1985 is ready to accept a ROM command. The
, minimum 480 s). The bus master then releases the line
bus master transmits (TX) a Reset Pulse (t
RSTL
and goes into receive mode (RX). The 1-Wire bus is pulled to a high state via the pullup resistor. After
, 15-60 s) and then transmits the
detecting the rising edge on the data pin, the DS1985 waits (t
PDH
, 60-240 s).
Presence Pulse (t
PDL
Read/Write Time Slots
The definitions of write and read time slots are illustrated in Figure 10. All time slots are initiated by the
master driving the data line low. The falling edge of the data line synchronizes the DS1985 to the master
by triggering a delay circuit in the DS1985. During write time slots, the delay circuit determines when the
DS1985 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 DS1985 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.
PROGRAM PULSE
To copy data from the 8-bit scratchpad to the EPROM Data or Status Memory, a Program Pulse of 12
volts is applied to the data line after the bus master has confirmed that the CRC for the current byte is
correct. During programming, the bus master controls the transition from a state where the data line is
idling high via the pullup resistor to a state where the data line is actively driven to a programming
voltage of 12 volts providing a minimum of 10 mA of current to the DS1985. This programming voltage
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