Rainbow Electronics DS1972 - Manual (Page 18)

DS1972: 1024-Bit EEPROM iButton
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1-Wire SIGNALING
The DS1972 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-Zero, Write-One, and Read-Data. Except
for the Presence Pulse, the bus master initiates all falling edges. The DS1972 can communicate at two different
speeds, standard speed, and Overdrive Speed. If not explicitly set into the Overdrive mode, the DS1972
communicates at standard speed. While in Overdrive mode the fast timing applies to all waveforms.
To get from idle to active, the voltage on the 1-Wire line needs to fall from V
PUP
below the threshold V
TL
. To get
from active to idle, the voltage needs to rise from V
ILMAX
past the threshold V
TH
. The time it takes for the voltage to
make this rise is seen in Figure 10 as ' e' and its duration depends on the pullup resistor (R
PUP
) used and the
capacitance of the 1-Wire network attached. The voltage V
ILMAX
is relevant for the DS1972 when determining a
logical level, not triggering any events.
Figure 10 shows the initialization sequence required to begin any communication with the DS1972. A Reset Pulse
followed by a Presence Pulse indicates the DS1972 is ready to receive data, given the correct ROM and memory
function command. If the bus master uses slew-rate control on the falling edge, it must pull down the line for
t
RSTL
+ t
F
to compensate for the edge. A t
RSTL
duration of 480µs or longer exits the Overdrive mode, returning the
device to standard speed. If the DS1972 is in Overdrive mode and t
RSTL
is no longer than 80µs. the device remains
in Overdrive mode. If the device is in Overdrive mode and t
RSTL
is between 80µs and 480µs, the device will reset,
but the communication speed is undetermined.
Figure 10. Initialization Procedure: Reset and Presence Pulse
RESISTOR MASTER DS1972
t
RSTL
t
PDL
t
RSTH
t
PDH
MASTER TX “RESET PULSE” MASTER RX “PRESENCE PULSE”
V
PUP
V
IHMASTER
V
TH
V
TL
V
ILMAX
0V
e
t
F
t
REC
t
MSP
After the bus master has released the line it goes into Receive mode. Now the 1-Wire bus is pulled to V
PUP
through
the pullup resistor, or in case of a DS2482-x00 or DS2480B driver, by active circuitry. When the threshold V
TH
is
crossed, the DS1972 waits for t
PDH
and then transmits a Presence Pulse by pulling the line low for t
PDL
. To detect a
Presence Pulse, the master must test the logical state of the 1-Wire line at t
MSP
.
The t
RSTH
window must be at least the sum of t
PDHMAX
, t
PDLMAX
, and t
RECMIN
. Immediately after t
RSTH
is expired, the
DS1972 is ready for data communication. In a mixed population network, t
RSTH
should be extended to minimum
480µs at standard speed and 48µs at Overdrive speed to accommodate other 1-Wire devices.
Read-/Write-Time Slots
Data communication with the DS1972 takes place in time slots, which carry a single bit each. Write-time slots
transport data from bus master to slave. Read-time slots transfer data from slave to master. Figure 11 illustrates
the definitions of the write- and read-time slots.
All communication begins with the master pulling the data line low. As the voltage on the 1-Wire line falls below the
threshold V
TL
, the DS1972 starts its internal timing generator that determines when the data line is sampled during
a write-time slot and how long data is valid during a read-time slot.