DS1982 Maxim, DS1982 Datasheet - Page 18

• Manufacturer Part Number: DS1982
• Description: The DS1982 1Kb 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

DS1982
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 pull–downs 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 DS1982 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 DS1982 is shown in Figure 10. A
Reset Pulse followed by a presence pulse indicates the DS1982 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 1-Wire line, the DS1982 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 11. All time slots are initiated by the
master driving the data line low. The falling edge of the data line synchronizes the DS1982 to the master
by triggering a delay circuit in the DS1982. During write time slots, the delay circuit determines when the
DS1982 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 DS1982 will hold the data line low overriding the 1 generated by the master. If
the data bit is a 1, the device 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 DS1982. This programming voltage
(Figure 12) should be applied for 480 s, after which the bus master returns the data line to an idle high
state controlled by the pullup resistor. Note that due to the high voltage programming requirements for
any 1-Wire EPROM device, it is not possible to multidrop non-EPROM based 1-Wire devices with the
DS1982 during programming. An internal diode within the non-EPROM based 1-Wire devices will
attempt to clamp the data line at approximately 8 volts and could potentially damage these devices.
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