DS2409 MicroLAN Coupler Design Alternative

Abstract: The DS2409 was originally designed to be used as a detection point (reader head) for access control and to reduce the bus load in large 1-Wire® networks. The DS2409 was also used to implement dual-master networks. However, Maxim has decided to discontinue the DS2409 and encourages all customers using the DS2409 to implement appropriate design modifications to cope with this situation. This application note describes alternatives to 1-Wire network systems using the DS2409 that do not require redesigning existing networks.

Overview

Thanks to the unique design of the DS2409 MicroLAN coupler , it can be used to implement a variety of special applications. This article first lists the features and applications of the DS2409, then introduces alternative circuits that can achieve the same functions. Finally, this article discusses these alternative circuits in detail.

DS2409 Features

The DS2409 is a special type of 2-port addressable switch device. Unlike traditional programmable input/output ports (PIOs) that output high or low levels, this device connects its output channel to the 1-Wire bus as its input through a transmission gate. At any time, only one 1-Wire output channel is valid. In addition to the 1-Wire output channel, the DS2409 also has a controllable output channel that can be used with the main output channel (default mode), with the auxiliary output channel, or independently. The switching of these functions is completed by controlling the status control byte (see the DS2409 data sheet). The status information byte (see the DS2409 data sheet) allows the host to confirm the device configuration status, check the device operating status (in use or not in use), confirm the logic state of each 1-Wire output (high or low), and the event flag bit (set/clear). Table 1 summarizes these features of the DS2409 and the convenience brought by these features.

Table 1. DS2409 Features and Benefits

The DS2409 requires a 5V VDD power supply. After power-on, all 1-Wire output channels are in a non-operating state and are pulled to the 5V VDD level through internal pull-up resistors. A short power failure will cause the device to enter a power-on reset state. After VDD stabilizes, a short low level is applied to the 1-Wire input port, causing the DS2409 to enter a software power-on reset state. When the DS2409 is reconnected to the host, the device enters the default power-on reset configuration state, the 1-Wire bus output channels are in a non-operating state, and the event flag bit state is uncertain.

The features listed in Table 1 are mainly for the following three applications: intelligent detection points, multi-layer networks, and dual-host networks (see Table 2).

Table 2. Characteristics and applications

Intelligent detection point

The block diagram shown in Figure 1 is an access control system that uses iButton® as an electronic key. R1/C1 should be installed as close to the 1-Wire adapter as possible. R1 is used to allow the DS2409 to perform a soft reset when the 1-Wire bus is interrupted. C1 prevents the DS2409 from blocking the 1-Wire bus when VDD overshoots.

Figure 1. Access control system topology with intelligent detection points

In addition to the main 1-Wire bus, probe points (such as DS9092 iButton probes) are located on the secondary network of branches from the main output of the DS2409. Connected to the auxiliary output is an ID chip, such as a 1-Wire EEPROM device, which stores the branch name. Under normal operating conditions, all branches are disabled (disconnected from the main 1-Wire bus) while the host performs a conditional search to detect whether an event has occurred. When an iButton is connected to a probe point, the DS2409 event flag on that branch is set. In the next scan cycle, the host will locate the DS2409 and enable the auxiliary output channel to read the branch name. Next, the host will access the same DS2409 and enable the main output channel to access the iButton connected to the system. When the main output channel enters the operating state, it lights up the LED to indicate to the user that a device connection has been detected. If the LED is not needed, the control output can be connected to a door lock and opened/closed under software control. Because the Smart-On command has a short-circuit detection function, the DS2409 can prevent network failure caused by short circuits at the probe point.

Multi-layer network

As with any network, it is important to find the best balance between the load driven by the master and the number of nodes (or branches, slaves). One way to achieve this is to layer the network (see Figure 2). The figure shows a 4-layer network, starting with the always-active trunk (layer 0), and each sublayer contains switching branches (layers 1 through 3). R1/C1 should be close to the 1-Wire master. If a multi-layer network is built with one or more DS2409s on the 1-Wire trunk and sublayers, R1 will ensure that the path is automatically closed when the master reconnects. C1 prevents the DS2409 from blocking the 1-Wire bus when VDD overshoots.

To effectively control this network, the host should know the ROM ID of each branch DS2409 slave in each layer to identify the network topology. To open the path to the layer 3 slave (as shown by the arrow), the host should execute the following command:

Execute the Match ROM command on trunk U1.

Enable the master output channel (this will open the path to both slave devices in Tier 1).

Execute the Match ROM command for layer 1 U3.

Enable the master output channel (this opens both slave paths in Layer 2).

Execute the Match ROM command for layer 2 U4.

Open the auxiliary output channel (this action will open the path for the slave device connected to this output at layer 3).

Figure 2. Multi-layer network topology

Since the DS2409 is powered from VCC, the maximum 1-Wire bus load is 100pF (50pF from the 1-Wire bus input and 50pF from the output), while the typical bus load for parasitic powered 1-Wire devices is 800pF to 1000pF. In the example topology, the total load that the master has to drive is: the DS2409 on the backbone (50pF), the two DS2409s on the first layer (150pF, 2 inputs, 1 output), the two DS2409s on the second layer (150pF), and all the slaves connected to the auxiliary output channels of U4 (50pF plus the slave load). The total load is therefore 400pF plus the slave load.

Depending on the application requirements, more than two DS2409s can be connected to each branch. Table 3 lists the maximum number of branches and the corresponding load from the coupler. From layer to layer, the number of branches increases exponentially, while the load from the DS2409 increases linearly.

Table 3. Multi-layer branch count and DS2409 load

In addition to the communication load that increases linearly with the number of network layers, users should also be aware of the impedance introduced by the DS2409. For the main output, the typical value is 10Ω (maximum 20Ω); for the auxiliary channel, the typical value is 15Ω (maximum 30Ω). Non-zero impedance reduces the high level of the final network layer (master to slave) and pulls up the low level of the backbone network (slave to master). Reducing the high level is usually not a problem, but because the low level threshold is pulled higher, it is usually recommended that users limit the network layering to 4 layers or less.

Dual host network

There are situations in applications where dual masters are needed to control the 1-Wire bus, for example, when a backup master is needed or when two masters need to communicate with each other. Figure 3 is a schematic diagram of the dual master implementation. In this example, the DS1996 memory iButton is used as a temporary register for data exchange. The ID chip, if installed, can store system-related information to inform the masters that they are accessing a shared network that also has data buffering and handshake logic. In addition to the memory iButton, there can also be a network consisting of 1-Wire slaves. It is recommended to use the R1/C1 combination shown in Figure 1 in a dual master network, and apply R1/C1 to the two master sides.

Figure 3. Dual host, if the same voltage cannot be guaranteed, the same power supply can be used to power both DS2409s.

As a starting point, both the primary and secondary output channels of the DS2409 are disabled. The common network is pulled up to 5V from the DS2409, reducing the equivalent pull-up resistance to 750Ω. Both masters periodically read the DS2409 to determine if another master has taken over the 1-Wire bus network.

Assume that Host A accesses the DS1996 and transmits data to Host B. To access the memory iButton (DS1996), Host A first turns on the controllable output of U1, which pulls the auxiliary output of U2 low. At the same time, Host B has read the status information of U2, so it knows that Host A has taken over the bus. Next, Host A turns on the main output of coupler U1 and writes data to the memory iButton. After these operations, Host A turns off the main output of U1 and turns off the control output.

Host B is still reading the status information of U2 and detects that Host A has completed the write operation. At this time, Host B turns on the control output of U2, which will pull the auxiliary output of U1 low. Host A reads the status information of U1 and knows that Host B has taken over the bus. At this time, Host B turns on the main output channel and reads back the data from the memory iButton. After completing the information processing, Host B writes a response message to the iButton. After this operation is completed, Host B turns off the main output of U2 and turns off the control output. Because Host A is still reading the status information of U1, it can know that Host B's access to the bus has ended.

Functional commands and their typical usage

The DS2409 has a total of 11 commands that implement network control functions. Table 4 lists these commands and explains their typical usage. These commands are ranked according to their importance in network applications. For more information, refer to the DS2409 data sheet.

Table 4. DS2409 function commands and their typical usage

Let's focus on the Smart-On command (Figure 4). The top waveform is the 1-Wire bus communication waveform, in this case the Smart-On Auxiliary command. The first byte is the 33h command code, followed by the reset signal FFh and the reset response (00h, indicating that a presence pulse was detected), followed by an acknowledgement byte (33h, indicating that there is no short circuit). The middle waveform shows the state of the auxiliary output channel, that is, the reset/presence detect (PD) cycle. The bottom waveform shows the waveform transition when the controllable output turns off the main output channel before turning on the auxiliary output. Any communication after the acknowledgement byte is carried out through the currently enabled channel. The presence pulse is only valid when it is preceded by an All Lines Off command.

Figure 4. Smart-On Auxiliary command

The All Lines Off command is usually used to turn off the output channel (Figure 5). The top waveform in the figure is the command byte 66h, followed by the confirmation byte. The command byte is output through the output channel (middle waveform), but the confirmation byte is not input from the channel. The bottom waveform shows the jump of the controllable channel level when turning off the output.

Figure 5. All Lines Off command

In addition to the Smart-On command, there is also a Direct-On command for the master output channel (Figure 6). The waveform on the oscilloscope is a mirror image of the All Lines Off command. After the command code A5h, the master output channel is turned on (bottom waveform). The confirmation byte is output from the output channel (middle waveform). If this command is used, a reset/acknowledge cycle must be generated to ensure that the slave on the turned-on channel is synchronized with the master.

Figure 6. Direct-On Main command

DS2409 Alternatives

To replace the DS2409, the user should use a 1-Wire addressable switch (for digital control and detection) and an analog switch (to turn the output on or off). A partial replacement for the DS2409 can be achieved with a two-channel addressable switch (such as the DS2413, DS2406, or DS28E04) and one or two analog switches. A complete replacement would require a 5-channel 1-Wire addressable switch (such as the DS2408, 8 channels) and two analog switches. Note that the addressable switch is powered up with all PIOs in the disconnected (non-conducting) state.

The analog switch must be a single-pole double-throw (SPDT), and the supply voltage should be 5V ±10%. The on-resistance (RON) should be less than or equal to 30Ω, the capacitance of the three switch nodes should be no more than 50pF, and the switching time should be less than 100ns. It is even better if high ESD protection is built in. Based on the above considerations, the following analog switches can be selected:

Single channel: MAX4729 (5.7Ω, max), MAX4730 (5.7Ω, max), MAX4644 (4.75Ω, max)

Dual-channel: MAX4717 (3.5Ω, max), MAX4719 (25Ω, max), MAX4635 (4.5Ω, max), MAX4636 (4.5Ω, max), MAX4750 (30Ω, max)

Three channels: MAX4693 (25Ω, typical; 40Ω, maximum, slow switching)

None of the above analog switches have high ESD protection. The single-channel switch MAX4561 is used on the test bench, and the normally open or normally closed pin has ±15kV ESD protection. However, since the RON of the MAX4561 is typically 45Ω, it is not suitable to replace the DS2409.

Example Circuit

The circuit shown in Figure 7 is a partial replacement circuit that implements 1-Wire output switching and output control of the DS2409. U1 is a 2-channel 1-Wire addressable switch (with open-drain PIO); U2 is a single-pole double-throw analog switch with three switch contacts corresponding to the NO, NC, and COM pins. The switch is controlled by the digital input level of the IN pin.

At power-on default, all 1-Wire addressable switch PIOs are in high impedance. Apply a high level to the switch IN pin through resistor R2, connecting the NO pin to the COM terminal. The NO pin is connected to the inactive 1-Wire output of the COM pin through a 1.5kΩ pull-up resistor R1. These configurations are equivalent to the power-on state of the DS2409.

To turn on the 1-Wire output on U2, the master turns PIO-A on, as if applying a low level to the IN pin of U2. This causes the COM terminal of the analog switch to switch from NO to NC, turning on the 1-Wire bus. Turning PIO-A off turns off the 1-Wire output. The master can also operate PIO-B independently of PIO-A, such as replacing the control output of the DS2409 in manual mode or using it to control other circuits such as R1/R2/U2. When controlling two analog switches, the 1-Wire master must ensure that only one 1-Wire output channel is turned on. This can be done in software or with safer connection logic, that is, by controlling the analog switches with PIO-A ^ /PIO-B and /PIO-A ^ PIO-B combinational logic. PIO-B can also be used for event detection (conditional search) and short detection (dashed line). However, it is not possible to send a reset pulse before the analog output is valid.

Figure 7. DS2409 partial replacement circuit. U1 can use DS2406, DS2413 or DS28E04.

DS2406 Addressable Switch

The circuit shown in Figure 7 has been tested with the DS2406 addressable switch. The IN pin of the analog switch (MAX4561) is connected to PIO-B of the DS2406. PIO-A is used to control the output, turning on the LED. The PIO is controlled by the Write Status command (code 55h) to control the memory 0007h (SRAM control bits), and bit 6 of the SRAM control bits directly controls the status of the PIO-B channel. Figure 8 shows the output enable process. The top waveform in the figure is the CRC16 byte (1Fh, E2h) following the 3Fh data byte written to the 0007h location. The bottom waveform is the transition waveform of PIO-B, which controls the analog switch. The 1-Wire bus output waveform (middle waveform) does not appear until the reset/acknowledge cycle after the CRC16 byte ends. Figure 8 does not show the reset/acknowledge cycle.

Figure 9 shows the process of turning off the output. The top waveform is the CRC16 bytes (1Eh, 12h) following the 7Fh data byte written to the 0007h location. The bottom waveform is the transition waveform of PIO-B, which controls the analog switch. After the PIO-B state transition, the 1-Wire output (middle waveform) is terminated, and then the master sends a reset/presence pulse. Figure 9 does not show the reset/presence cycle.

In addition to writing commands to the status register to change the PIO state, the PIO can also be controlled by the Channel Access command (code F5h), but this operation is not described in this article. Also note that the DS2406 performs a power-on reset operation less than 1 minute after power is removed. The DS2409 performs a power-on reset operation approximately a few milliseconds after power is removed or the 1-Wire input is disconnected (low). The DS2406 latches its PIO state, thereby supporting event detection (conditional search) and short-circuit detection (dashed line). The latched state is cleared by the Channel Access command (channel control byte 1).

Figure 8. Partial replacement using the DS2406, valid output

Figure 9. Partial replacement using the DS2406, output disabled.

DS2413 Addressable Switch

The DS2413 addressable switch was tested using the circuit shown in Figure 7. The IN pin of the analog switch is connected to PIO-A of the DS2413. PIO-B is used to control the output and illuminate the LED. The PIO is operated by the PIO Access Write command (code 5Ah), and the output is enabled as shown in Figure 10. The top waveform is the PIO output data byte (first true FEh, followed by inverted 01h), followed by the AAh acknowledgement byte, and the new PIO pin state (3Ch). PIO-A (bottom waveform) is used to control the analog switch, and its state changes after the inverted PIO output data byte. Therefore, the acknowledgement byte and PIO pin state appear on the 1-Wire output channel (middle waveform). To ensure that the slaves on the bus are synchronized with the master, the master must issue a reset/presence pulse.

Figure 10. Partial replacement using the DS2413, valid output

Figure 11. Partial replacement using the DS2413, output disabled.

Figure 11 shows the output disable waveforms. The top waveform is the PIO output data byte (first true FFh, followed by the inverted 00h). The acknowledge byte and the new PIO pin state (3Ch) are not shown in the figure. PIO-A (bottom waveform) is used to control the analog switch, and its state changes after the inverted PIO output data byte. The acknowledge byte and PIO pin state are not output on the 1-Wire (middle waveform).

Note that the DS2413 performs a power-on reset after being disconnected from the 1-Wire bus for 5 minutes. The DS2406 performs a power-on reset within 1 minute. The DS2413 does not latch the pin states, so it does not support conditional searching, but it does implement short-circuit detection (dashed line).