Wirelessly Powering and Accessing a 1-Wire Network
Initial Charging of a 1-Wire Network
1-Wire® devices power-up parasitically by charging an internal reservoir from the 1-Wire communication line. VPUP, the pull-up voltage on the 1-Wire network, is dependent upon VOUT and RPUP. The maximum current that can be supplied to the 1-Wire network also depends on these two parameters.
When powering a 1-Wire network from a high-impedance source like the MAX66242 voltage regulator at VOUT, before sending 1-Wire function commands, take care to ensure that enough time passes before the devices attached to the 1-Wire network are charged and ready to communicate. This occurs when the initial capacitance CIO at its I/O pin is charged.
Most 1-Wire devices specify typical and maximum CIO values that exist on their 1-Wire I/O port. The maximum CIO exists when VPUP is first applied to the 1-Wire network. After the 1-Wire network is fully charged, only the typical CIO affects 1-Wire communication. Therefore, the CIO-MAX should be charged to the minimum pull-up voltage VMIN-PUP required by the 1-Wire device. Equation (1) defines the minimum pull-up voltage VMIN-PUP across the total maximum capacitance CTOTAL-MAX of the 1-Wire network.
VMIN-PUP = VS(1 - etCHARGE/ -RS+PUPCTOTAL-MAX ) [Eq. 1]
where CTOTAL-MAX = ΣNi=0 (CMAX-IO, i + CLAYOUT), RS+PUP = RS + RPUP, VMIN-PUP is the minimum pull-up voltage required on the 1-Wire network, and VS is the open-circuit voltage at VOUT. The capacitance CLAYOUT represents the capacitance introduced to the 1-Wire network because of junctions on the 1-Wire node (See Figure 1 for a depiction.)
Figure 1. 1-Wire network modeled as a series of I/O capacitances CIO and parasitic layout capacitance CLAYOUT as a result of junctions on the 1-Wire node.
VMIN-PUP is the largest minimum-pull-up voltage in the 1-Wire network. So, if device number one has a minimum pull-up voltage of 2.8V and device number two has a minimum pull-up voltage of 3.0V, then VMIN-PUP should equal to 3.0V for the 1-Wire network.
Equation (2) determines the time tCHARGE necessary to charge the total maximum capacitance CTOTAL-MAX to the minimum pull-up voltage VMIN-PUP of a 1-Wire network:
tCHARGE = -RS+PUPCTOTAL-MAXln(1 – VMIN-PUP / VS) [Eq. 2]
Parasitic Capacitance During 1-Wire Communication
The total typical capacitance CTOTAL-TYP on the 1-Wire network after powering up is defined as the sum of all typical capacitances CTYP-IO plus the parasitic capacitance of the layout CLAYOUT. This is represented in schematic form in Figure 9 by replacing CMAX-IO,N with CTYP-IO,N where CTOTAL-TYP = ∑ni=1CTYP-IO, i + CLAYOUT. The typical capacitance CTYP-IO refers to the parasitic capacitance at the I/O that originates from each of the device’s internal 1-Wire receivers/transmitters. In every 1-Wire communication sequence, the typical capacitance CTYP-IO, the pull-up voltage VPUP, and the pull-up resistance RPUP are responsible for the following four fundamental timing parameters:
- ε – the time taken to pull up from 0V to the 1-Wire network’s threshold-high voltage VTH.
- δ – the time taken to pull up from 0V to the 1-Wire host input-high voltage VIH-HOST.
- tREC – the time taken to pull up from VTH to VPUP. tREC defines the maximum time available for the 1-Wire network to recharge during communication.
- tf – the time taken to pull down from VPUP to the 1-Wire network’s threshold-low voltage VTL.
Time constants ε, δ, tREC, and tf help specify a maximum total typical capacitance CTOTAL-TYP that can allow proper 1-Wire communication for a given RPUP and VPUP. If CTOTAL-TYP is exceeded, then timing constraints are not met, rendering 1-Wire communication improbable. For the value of the four time constraints, see the datasheet for the respective 1-Wire device.
Pull-up Fundamental Timing Parameters ε, δ, and tREC
Figure 2 illustrates time ε needed to charge up the 1-Wire total typical capacitance CTOTAL-TYP from 0V to VTH. Figure 3 presents this concept in schematic form.
Figure 2. Time ε to charge the total typical capacitance CTYP-TOTAL from 0V to VTH.
Figure 3. 1-Wire network modeled as an equivalent total typical capacitance CTOTAL-TYP that includes the parasitic layout capacitance CLAYOUT.
Equation (3) defines ε – the time required to charge CTOTAL-TYP from 0V to VTH via RS+PUP.
ε = -RS+PUPCTOTAL-TYPln(1 – VTH / VS) [Eq. 3]
Figure 4 illustrates time δ needed to charge up the 1-Wire total typical capacitance CTOTAL-TYP from 0V to VIH-HOST.
Figure 4. Time δ to charge total typical capacitance CTYP-TOTAL from 0V to VIH-HOST.
Equation (4) defines ε – the time required to charge CTOTAL-TYP from 0V to VTH via RS+PUP.
δ = -RS+PUPCTOTAL-TYPln(1 – VIH-HOST / VS) [Eq. 4]
Figure 5 illustrates the shortest time tREC needed to recharge the 1-Wire total typical capacitance CTOTAL-TYP from VTH to VPUP-MIN.
Figure 5. Shortest time tREC possible to charge total typical capacitance CTYP-TOTAL from VTH to VPUP-MIN.
Follow this three-step procedure to calculate tREC:
- Calculate the time required to charge from 0V to VTH – this is equivalent to ε in Equation (3).
- Calculate the time required to charge from 0V to VPUP-MIN.
t’ = -RS+PUPCTOTAL-TYPln(1 – VPUP-MIN / VS)
- Use the quotient rule to find tREC = t’ – ε.
tREC = RS+PUPCTOTAL-TYPln[(1 – VTH/VS) / (1 – VPUP-MIN/VS)] [Eq. 5]
Pull-Down Timing Parameter tf
Unlike ε, δ, and tREC, time tf does not depend on RS and RPUP because time tf defines the time required for the 1-Wire host or device to pull down the 1-Wire network. Therefore, the pull-down resistance RPDOWN of the 1-Wire host or device defines the time tf necessary to discharge CTOTAL-TYP from VPUP to VTL as illustrated in Figure 6.
Figure 6. Time tf to discharge total typical capacitance CTYP-TOTAL from VPUP-MIN to VTL.
The pull-down resistance RPDOWN for the 1-Wire host and device comes from the maximum output low-voltage VOL and the corresponding output low-current IOL provided in the electrical characteristics table of the respective datasheet. Figure 7 illustrates the pull-down resistance RPDOWN and the pull-down current IOL.
Figure 7. Simplified RC circuit that models the pull-down resistance RPDOWN from either the 1-Wire host or device. IOL is the pull-down current.
For example, the DS2484 I2C-to-1-Wire bridge has a maximum VOL of 0.4V at 4mA. This means that the maximum pull-down resistance is RPDOWN is 100Ω.
Equation (6) defines the discharge time tf.
tf = -RPDOWNCTOTAL-TYPln(VTL / VPUP-MIN) [Eq. 6]
Note that time tf should be met for the 1-Wire host and the device. The 1-Wire host pulls down the 1-Wire node at the beginning of each basic operation, i.e., reset, write 1 bit, write 0 bit, and read a bit. The 1-Wire device pulls down the 1-Wire node during a reset operation to generate a presence pulse.
1-Wire communication and power delivery are possible when the four fundamental parameters ε, δ, tREC, and tf are met for all devices on the network. You can determine the maximum number of devices and bus length achievable in the NFC-powered system by knowing the total allowable capacitance to meet all edge timings specified by the 1-Wire protocol.
For more information about ε, δ, and tREC, and how they affect 1-Wire communication in standard or overdrive mode, refer to Application Note 126 – 1-Wire Communication Through Software. For more information about how typical capacitance affects 1-Wire communication, refer to Application Note 148 – Guidelines for Reliable Long Line 1-Wire Networks.
Compatible 1-Wire Devices
Table 1 lists 1-Wire devices with their respective input/output capacitance CIO, pull-up voltage VPUP, pull-up resistance RPUP, voltage threshold-low VTL, and voltage threshold-high VTH specifications. VTL is the voltage below which, during a falling-edge on the 1-Wire network, a logic low is detected. VTH is the voltage above which, during a rising-edge on the 1-Wire network, a logic high is detected. Both VTL and VTH are a function of VPUP and 1-Wire recovery times.
|Device||CIO (pF)||VPUP (V)||RPUP (kΩ)||VTL (V)||VTH (V)|
|DS1925 High-Capacity Temp. Logger w/ 122kb Data-Log Mem.||120000||3.0||5.25||2.2||0.5VPUP||0.75VPUP|
|DS1923 Hygrochron Temperature/Humidity Logger||100||800||3.0||5.25||2.2||0.4||3.2||0.7||3.4|
|DS1922 High Temp. Logger w/ 8kb Data-log Mem.|
|DS1922L/T Temp. Logger w/ 8kb Data-log Mem.|
|DS1921H/Z High Resolution Thermochron|
|DS1904 Real-Time Clock||50||2.8||6.0||5 (Note 1)||0.8||2.2||6.0|
|DS1992/3 Memory||100||800||2.8||6.0||5 (Note 1)||0.3||2.2|
|DS1990R Serial Number||100||800||2.8||6.0||0.6||5||0.3||2.2|
|DS1977 Password-Protected 32kb EEPROM||5000||2.8||5.25||0.6||2.2||0.5||3.2||0.7||3.4|
|DS1982/DS9105 Memory||800||2.8||6.0||5 (Note 1)||0.8||2.2|
|DS1996||100||800||2.8||6.0||5 (Note 1)||2.2|
|DS1920 Temperature Logger||800||2.8||6.0||0.8||2.2|
|DeepCover® Secure Authenticators with SHA-256|
|DS28E15 512b User EEPROM||1500||2.97||3.63||0.3||1.5||0.65VPUP||0.75VPUP|
|DS28E22 2kb User EEPROM|
|DS28E25 4kb User EEPROM|
|DeepCover® Secure Authenticators with ECDSA|
|DS28E35 1kb User EEPROM||1500||2.97||3.63||0.3||1.5||0.65VPUP||0.75VPUP|
|DS24B33 4kb EEPROM||2500||2.8||5.25||0.3||2.2||0.5||VPUP -1.8||1.0||VPUP -1.0|
|DS2413 Addressable Switch||100||800||2.9||5.25||1.5||2.2||0.4||3.2||0.7||3.6|
|DS28E04-100 Switch, EEPROM, PIO||100||800||2.8||5.25||0.3||2.2||0.46||4.40||1.0||4.9|
|DS2408 Addressable Switch||1200||3.3||5.25||2.2||0.5||3.2||0.8||3.4|
|DS2431 1kb EEPROM||1000||2.8||5.25||0.3||2.2||0.5||VPUP-1.8||1.0||VPUP-1.0|
|DS2430A 256-bit EEPROM||1000||2.8||5.25||0.3||2.2||0.46||VPUP-1.8||1.0||VPUP-1.1|
|DS2401 Serial Number||800||2.8||6.0||1.5||5.0||0.3||2.2|
|DS2406 Switch w/ 1kb Mem.||800||2.8||6.0||0.5||2.2|
|DS28E80 Radiation Resistant||6500||2.97||3.63||300||750||0.65VPUP (Note 1)||0.75VPUP (Note 1)|
|DS28E05 112-byte EEPROM||1500||1.71||3.63||0.3||750|
|DS28E05 112-byte EEPROM||1000||3.0||5.25||0.3||2.2|
|DS28EA00 Sequence Detect and PIO||1000||3.0||5.5||0.3||2.2||0.46||VPUP-1.9||1.0||VPUP -1.1|
|DS1825 with 4-bit address||25||3.0||3.7||4.7 (Note 1)||0.7||3.0|
|DS18S20-PAR||25||3.0||5.5||4.7 (Note 1)||0.8||3.0|
|Timekeeping with Interrupt|
By modeling an NFC transponder connected to a 1-Wire network as an RC circuit, we can verify whether harvested power delivery and communication are feasible. A smartphone or any device equipped with an NFC transceiver under ISO15693 and FIPS180-4 can authenticate, identify, access memory from, conduct data acquisition on, and control a 1-Wire network. With an NFC system, we can wirelessly power a 1-Wire network and allow secure asset and information management for a node of closed mobile systems and internet of things (IoT) devices.
A similar version of this application note originally appeared on Power Electronics on May 30, 2018.