Abstract

The DS2480B is a 1-Wire^® master (driver) with UART host interface. This driver is optimized for power delivery and supports overdrive speed in embedded applications. A special feature of the DS2480B is its flexible speed mode, which allows the designer to configure the 1-Wire timing at standard speed. This application note explains how to determine the optimum timing configuration settings and how to write the settings to the chip using Windows® software. The article also compares the driver strength of the DS2480B to a resistive pullup, as described in application note 3829. The first appendix explains how the optimum set of configuration parameters was determined. A second appendix describes an algorithm to estimate the number of slaves that the DS2480B can drive, depending on the electrical characteristics of the master and the capacitive load of the network cabling. The third appendix discusses network overload conditions.

Introduction

The DS2480B is a 1-Wire master (driver) with UART host interface. Optimized for power delivery and supporting overdrive speed in embedded applications, it relieves the host from the duty of generating time-critical 1-Wire waveforms. This document discusses the configurability and performance of the DS2480B. For guidance on host software development including flow charts, pseudo code, and a 'C' implementation see application note 192, "Using the DS2480B Serial 1-Wire® Line Driver."

The DS2480B uses active circuitry to shorten the recovery time at the end of time slots. Figure 1 shows a simplified schematic of the 1-Wire driver section.

Figure 1. Simplified drawing of the DS2480B 1-Wire driver circuit.

When the 1-Wire line is idle, the DS2480B driver performs the pullup through a controlled current source. This current source can be switched off (during pulldown), provide weak pullup (I_WEAKPU after pulldown and when idle), or provide an active pullup (I_ACTPU during rising edges). The pulldown circuit (Q1) implements a slew rate that can be adjusted through software. Q2 represents the power delivery circuit used by 1-Wire slave functions that require high current, such as EEPROM programming or performing temperature conversion. The function of Q2 is not discussed in this application note.

To support configurations with significant cabling, an external line termination resistor, R_T (~100Ω), is needed to minimize transmission line effects such as reflections, undershoot, and overshoot. However, a consequence of adding R_T is that the active pullup can turn off prematurely before the 1-Wire line is fully recharged at the slave device(s). This can happen because the DS2480B senses voltage at its 1-Wire pin rather than the slave side of the 1-Wire network. The slave side will be lower than the 1-Wire pin by the voltage drop across R_T when the pin reaches an internal threshold (typically I_ACTPU × R_T = 15mA × 100Ω = 1.5V). Crossing this threshold starts a timer that must expire before the active pullup is turned off. But since the timer starts early, it also ends early. Nevertheless, after the active pullup turns off (early), the 1-Wire line will continue to charge with the weak pullup current, I_WEAKPU, until it is fully recharged.

Critical Parameters

In very simple terms, a 1-Wire network can be compared to a lossy capacitor that is charged and discharged as communication occurs. Consequently, anything that affects the process of charging and discharging the network is a parameter that must be considered when determining the driver performance. The critical hardware-defined parameters that characterize the DS2480B driver are:

• 1-Wire Weak Pullup Current, I_WEAKPU
• 1-Wire Active Pullup Current, I_ACTPU
• Active Pullup on Threshold, V_IAPO
• Active Pullup Timer Threshold, V_IAPTO
• Active Pullup on Time, t_APUOT (2.0µs at standard speed, 0.5µs at overdrive)

1-Wire Master Timing Configurability

The DS2480B offers flexible speed mode, which is a variant of standard speed. This special flexible speed mode allows fine-tuning of the timing for the 1-Wire time slots. The timing of a reset-/presence-detect sequence is fixed. Timing parameters that can be set by writing to control registers are:

• Write 1 Low Time (t_LOW1)
• Data Sample Offset (t_DSO)
• Write 0 Recovery Time (t_REC0)
• Slew Rate of Falling Edges (DS2480B initiated)

Table 1. Variable Parameters: Flexible Speed Only

As illustrated in Figure 2, the Write 1 and Read Data time slots are constructed from three segments: t_LOW1, t_DSO, and t_HIGH1. The duration of t_LOW1 can be set from 8µs to 15µs. With a t_DSO range from 3µs to 10µs and a fixed t_HIGH1 of 49µs, the duration of a Write 1 or Read Data time slot can be anywhere from 60µs to 74µs. The sampling to read from the 1-Wire (in case of a Read time slot) takes place at t_LOW1 + t_DSO after the beginning of the time slot, i.e., anywhere from 11µs to 25µs from the beginning of the falling edge.

Figure 2. Write 1 and Read Data time slots.

A Write 0 time slot (Figure 3) consists of the two segments, t_LOW0 and t_RECO. With a fixed t_LOW0 of 57µs and a t_RECO range from 3µs to 10µs, a Write 0 time slot can last from 60µs to 67µs. Depending on the data that the DS2480B receives from the host's UART, there can be significant idle time between time slots.

Figure 3. Write 0 time slot.

The instant at which a 1-Wire slave recognizes a time slot depends on three factors: the slew rate of the falling edge of the master pulldown; the slave's electrical characteristics; and the physical distance between master and slave. In flexible speed mode the slew rate can be adjusted from a fast 15V/µs down to a slow 0.55V/µs. A very slow slew rate can, however, conflict with a short t_LOW1 setting. Extensive tests have shown that a slew-rate setting of 1.37V/µs is a good starting point, and works in almost all applications with and without line termination. The fastest setting (15V/µs) should be avoided unless the line is properly terminated. Without termination, there is significant likelihood of ringing, particularly if the DS2480B drives a long cable.

How to Change the Settings

As shown in the Appendix A, "Choosing Configuration Parameter Values," the optimal configuration is a time slot duration of 66µs with t_LOW1 = 8µs and t_DSO = 9µs.

An executable Windows utility program, tmline.exe, to change the settings can be downloaded (ZIP). This file must first be extracted from the ZIP folder and moved to the Windows desktop. To run the program, double-click its icon or right click the icon and select "open." The program opens a DOS-like window and performs several tasks: introduces itself; identifies the revision of the underlying driver software; and displays the default settings for Pulldown Slew Rate, Write 1 Low Time, and Data Sample Offset/Write 0 Recovery Time. If 1-Wire application software is already running at the time where the tmline program is started, the displayed settings are the current settings.

Current (STANDARD) 1-Wire line settings:
 PDSR  = 1.37 V/us
 W1LT  = 8 us
 DSOW0 = 6 us

Select the PDSR (pulldown slew rate):
 0) 15 V/us
 1) 2.20 V/us
 2) 1.65 V/us
 3) 1.37 V/us
 4) 1.10 V/us
 5) 0.83 V/us
 6) 0.70 V/us
 7) 0.55 V/us
 8) EXIT

Enter number: 3

After a valid number is entered, the next parameter to be specified is Write 1 Low Time.

Select the W1LT (write-1 low time):
 0) 8 us
 1) 9 us
 2) 10 us
 3) 11 us
 4) 12 us
 5) 13 us
 6) 14 us
 7) 15 us
 8) EXIT

Enter number: 0

After a new Write 1 Low Time is entered, the last parameter for user input is Data Sample Offset/Write 0 Recovery Time.

Select the DSO/WOR (data sample offset/write 0 recovery):
 0) 3 us
 1) 4 us
 2) 5 us
 3) 6 us
 4) 7 us
 5) 8 us
 6) 9 us
 7) 10 us
 8) EXIT

Enter number: 6

After a valid number is entered, tmline writes the new set of parameters to the DS2480B in the port adapter. It then loops back to display the new settings as the current ones and asks for a new pulldown slew rate. To exit the program, enter "8." The new setting remains valid as long as the 1-Wire application software is running.

Recovery Time for Multiple Slave 1-Wire Networks

Standard Speed Mode

For a single-slave network in standard speed mode, the 1-Wire slave data sheets typically specify a minimum recovery time of 5µs in conjunction with a 2.2kΩ pullup resistor and a minimum pullup voltage of 2.8V. Maxim's application note 3829, "Determining the Recovery Time for Multiple-Slave 1-Wire Networks," explains how to determine the recovery time for multiple slaves, four temperatures, and two voltages. Since the DS2480B is a 5V device, the 4.5V section of application note 3829 is relevant. For temperatures of -5°C and warmer, the recovery time at standard speed is specified as 2 × N + 1µs (N = number of slaves). Accordingly for one slave, one needs 3µs, for two slaves 5µs, for three slaves 7µs, and for four slaves 9µs.

Therefore, using the calculations for worst-case conditions (1.5mA I_WEAKPU, 7mA I_ACTPU) from the Appendix B and the configuration of "four slaves with 9µs" as references, the DS2480B can handle at least 26 slaves. Consequently, at standard speed the DS2480B driver is at least six and a half times as strong as the 2.2kΩ passive pullup.

Overdrive Speed Mode

For a single-slave network in overdrive speed mode, the 1-Wire slave data sheets typically specify a minimum recovery time of 2µs in conjunction with a 2.2kΩ pullup resistor and a minimum pullup voltage of 2.8V. According to application note 3829 cited above, for -5°C temperatures and warmer, the recovery time at overdrive speed is specified as 1.37 × N + 0.5µs (N = number of slaves). Accordingly, for one slave one needs 1.87µs, for two slaves 3.24µs, for three slaves 4.61µs, and for four slaves 5.98µs.

Therefore, by using the condition "two slaves with 3.24µs" as a reference and scaled to 1.8 slaves at 3µs, and then by performing the same calculation for a worst-case condition at overdrive speed, the DS2480B can handle at least seven slaves. Consequently, at overdrive speed the DS2480B driver is at least four times as strong as the 2.2kΩ passive pullup.

What Happens If the Number of Slaves Exceeds the Recommendations?

The answer to the above question is straightforward: the network will probably continue functioning. This is true because:

• V_IAPTO is usually higher than the worst case assumed for the calculation.
• The slaves tolerate incomplete recharge as long as the voltage rises high enough to be recognized as the end of the time slot.
• V_IH1 of the DS2480B is probably lower than the minimum specification in the data sheet.

Summary

The reliability of a 1-Wire network depends equally on master (driver) and slaves. For operation at standard speed, the DS2480B 1-Wire driver's optimal setup is flexible speed with t_LOW1 = 8µs, t_DSO = t_RECO = 9µs. The slew-rate setting of 1.37V/µs is a good start. This setup accommodates 1-Wire slaves in the speed range of 15µs to 54µs. A single slave with a time base slower than 54µs can make the system nonfunctional. Changing the pulldown slew rate to 1.65V/µs or 2.2V/µs increases the chance for a "borderline" slave to function in the network. To accommodate 1-Wire slaves as slow as 60µs, the duration of t_HIGH1 and t_LOW0 must be increased by 6µs to 55µs and 63µs, respectively. This latter capability, however, is not yet available with the current revision of the DS2480B.

As an example, consider the free OneWireViewer which uses the 1-Wire drivers for Windows. The OneWireViewer software uses the configuration t_LOW1 = 8µs, t_DSO = 6µs, and a slew rate of 1.37V/µs as default for standard speed. As explained above, it might be necessary to increase t_DSO to 9µs. OneWireViewer revisions 4.01 and newer changed the default t_DSO to 9µs, thus matching the recommendation in this application note. The defaults for t_LOW1 and the slew rate remain at 8µs and 1.37V/µs, respectively.

The DS2480B is a strong 1-Wire driver. At standard speed the DS2480B can drive at least six and a half times and at overdrive speed four times as many slaves as a circuit that relies on a 2.2kΩ pullup resistor.

Appendix A

Choosing Configuration Parameter Values

Comprised of the three segments, t_LOW1, t_DSO, and t_HIGH1, the Write 1 and Read Data time slots can have 64 variations, as shown in Table 3. The longer t_DSO is (alias t_RECO for Write 0 times slots), the more time is available for network recharge (recovery). A combination of maximum settings for t_LOW1 and t_DSO, however, positions the sampling point up to 25µs into the time slots. The sampling point is where the DS2480B reads from the 1-Wire line.

Table 3. Time Slot Duration as Function of t_LOW1 and t_DSO

What happens if the Write 1 and Read Data time slot is different from 66µs? A shorter duration is acceptable for fast 1-Wire slaves. However, for every µs less, the slowest slave that can be accommodated must be 1µs faster. In the extreme case of a 60µs time slot, the slowest slave that would still function in the network must not be slower than 54µs. A time-slot duration longer than 66µs is acceptable for slow 1-Wire slaves. However, for every additional µs, the fastest slave must be 1µs slower to remain readable in the network. In the extreme case of a 74µs time slot, the fastest slave that would still function must not be faster than 23µs. There is a small chance that some randomly chosen 1-Wire slaves do not meet this last condition. Therefore, time-slot choices of less than 64µs or more than 68µs are less reliable. Most 1-Wire slaves have a speed close to the center of the 15µs to 60µs range, and devices slower than 54µs are rare.

Since the value chosen for t_DSO also applies to t_RECO for recharge in Write 0 time slots, the highest possible value for t_RECO is desirable. This high value identifies the pair t_LOW1 = 8µs and t_DSO = 9µs as the optimal configuration (highlighted in Table 3). Reviewing the Write 0 time slot from a slave's perspective, the slave might start its timer ~3µs into t_LOW0 and read (sample) 60µs later. Since t_LOW0 ends 57µs after the beginning of the time slot, the slave's sampling occurs 6µs late, near the end of t_RECO. To guarantee that a slave does not miss the latest sampling point in a Write 0 time slot, its time base must not be slower than 54µs. This requirement also ensures that the shortest recovery time in a Read 0 case is the same as in the Write 0 time slot, i.e., 9µs.

Appendix B

How Many Slaves Can the DS2480B Drive?

There is no simple answer to this question. A 1-Wire slave does not behave like an ideal capacitor. However, to describe the behavior one can use a capacitor model with a low capacitance until a certain voltage and a high capacitance above this voltage. The high capacitance corresponds to the parasitic supply capacitor and applies when the slave's recharge begins.

To speed the recharge process, the DS2480B uses a 2-stage current source instead of a resistor. The recharge begins with the low I_WEAKPU and later changes to the stronger I_ACTPU. The detailed view of a rising edge in Figure 4 illustrates the process.

Figure 4. Rising edge details.

At t₁ the pulldown (master or slave) ends and the weak pullup current begins charging the 1-Wire line. The load on the bus and the I_WEAKPU value of the DS2480B determine the slope. At t₂ the voltage crosses the threshold voltage V_IAPO. Now the DS2480B switches over to I_ACTPU, which is significantly higher. Consequently, the voltage on the 1-Wire line rises faster. As the voltage crosses the threshold V_IAPTO at t₃, a timer is started, which keeps the I_ACTPU flowing for another 2µs (t_APUOT, flexible speed) or 0.5µs (overdrive speed). After the timer expires, the I_WEAKPU continues to power the 1-Wire bus until the DS2480B begins the next time slot or communication sequence.

The charging of a capacitor using a constant current source can be described as:

V(t) = ICHARGE × t/C

(Eq. 1)

t2 = C × VIAPO/IWEAKPU

(Eq. 2)

t3 = C × [(VIAPTO - VIAPO)/IACTPU + VIAPO/IWEAKPU]

(Eq. 3)

After completing the above calculations, the resulting number of slaves for flexible speed is 26. Note that this number is a conservative estimate. The actual achievable number of slaves could be significantly higher. The input parameter values in Table 4 were used to calculate the result.

Table 4. Values to Calculate Number of Slaves for Flexible Speed

Table 5. Values to Calculate Number of Slaves for Overdrive Speed

Note that these calculations do not account for the capacitance of the cable that connects master and slaves. To obtain the number of slaves that can be supported for a given cable, calculate the cable capacitance and add it to the total slave capacitance when calculating the duration of Phase 1 and the voltage increments achieved during Phases 2 and 3.

Appendix C

Network Overload Conditions

There can be two types of overload on a 1-Wire line, capacitive and DC. A capacitive overload appears as insufficient recovery time. It is typically seen as a spike at the end of a Write 0 time slot, i.e., when the next time slot begins before the recharge is completed. A capacitive overload is unlikely if the setup and load follow the recommendations in this application note.

DC overload occurs if there is a resistor of ~3kΩ or less connected between the 1-Wire and GND. The extra "leakage" causes the 1-Wire voltage during recovery times to drop below the driver's supply voltage. If this occurs, DC overload is visible on rising edges, as shown in Figure 5. After having reached V_DD level, the voltage on the 1-Wire line drops to a lower level as soon as the I_ACTPU ends.

Figure 5. DC overload visible on rising edge.