In the first part of this series we took a closer look at the basics of LIN networking, the key parameters for a two-wire LIN (Atmel) solution and the details of a LIN Bus power supply. In the second part of this series, we discussed various aspects of slave node current consumption, specifically, system clock frequency, sleep mode power management and LIN scheduling power management. In the third part of this series, we got up close and personal with slave node buffer capacitance, LIN Bus data protocol and a multi-slave evaluation network.
And today we’re going to explore a network start-up and the voltage regulator, a four node network (1 master, 3 slaves) and a five node network (1 master 4 slaves). Essentially, a multi-slave, two-wire LIN network can be implemented so long as the supply voltage to the slave node does not drop below the 5.5V minimum input voltage requirement of the ATA6617 voltage regulator. In this regard, extensive testing has shown that the network as currently configured cannot support more than three slave nodes at any one time.
Meaning, the effective load placed upon the LIN master pull-up resistor simply cannot source enough current to meet the minimum input voltage requirement under all operating conditions. Ultimately, the network is limited by the voltage drop across the LIN master pull-up resistor and the cumulative load induced by the multiple slave nodes. Adding slave nodes to the network will increase the effective load placed upon LIN master pull-up resistor.
“Simply put, the load placed upon Vbatt results in an increased voltage drop across the master pull-up resistor, RLIN, thus decreasing the input supply voltage to the slave nodes. If the input voltage falls below 5.5V, the minimum input voltage required for ATA6617 voltage regulator operation, the output will become unregulated and the slave node(s) will be rendered inoperable,” Atmel engineering rep Darius Rydahl told Bits & Pieces.
“In this mode of operation, the voltage regulator pass transistor behaves as a switch and the input voltage flows directly through to the regulator output. Voltage regulator current in this region is unstable and can be upwards of 3mA in excess of the normally regulated current. Operation in this unstable region will lead to non-linear increases in the voltage drop across the LIN master pull-up resistor, RLIN.”
As such, says Rydahl, increasing the number of slave nodes on the network greatly raises the risk that an “unregulated” voltage regulator condition will occur. This is due to the brief, but instantaneous spike in the load current of each slave node when power is initially supplied to the network at start-up.
Extra current is required to kick-start the voltage regulator of each slave node. Even though the average current consumption in the multi-slave network is approximately 0.8mA per slave node, an extra 2mA to 3mA of current must be factored into the overall current consumption of each node at start-up.
In terms of a four-node network (1 Master, 3 Slaves), figure 10 shows the effect that the load has upon the LIN bus line at network power-on when three slave nodes are connected to the network. The plot clearly shows that slave node start-up briefly places an extra load on the network not seen during normal operation. At start-up, the LIN bus supply voltage hovers around 5.5V. Eventually, the slave node voltage regulators stabilize and the supply voltage settles to 8.2V. Network communication begins at this point.
“On the five-node network (1 Master, 4 Slaves) side, figure 11 shows the start-up behavior when a fourth slave node is added to the network. In this case, the LIN bus supply voltage is never able to recover from the start-up load condition and hovers at 5V (0.5V below the minimum operating voltage of the voltage regulator). The measured voltage drop across the LIN master pull-up resistor in this case is 3.3V,” Rydahl continued.
“The load current through the 220Ω LIN master pull-up resistor under these conditions is calculated by: I RLIN = VRLIN / RLIN = 3.3 / 220 = 15mA. Referencing the plot from figure 3, one can see that the maximum load current supported by the 220Ω LIN master pull-up resistor is approximately 13mA at 5.5V. The 15mA load caused by the addition of the fourth slave node is 2mA greater than the two-wire LIN network can handle.”
As a result, the slave nodes fail to respond to the master frame requests. To mitigate this effect, consider the scenario where the slave nodes are started sequentially (one node after the other, not all at once). In this case, network communication will occur as shown in figure 12. Staggering the start-up of the individual slave nodes greatly reduces the current load on the network at reset, in effect increasing the node handling capabilities of the two-wire network.
A network using this implementation could potentially run up to 12 slave nodes under the same network conditions; a) current per slave node is 0.8mA and b) 3mA voltage regulator start-up transient is limited to one slave node at a time. Then, I Slave_total = number of slaves × ISlave = 12 × 0.8 = 9.6mA and I Network = ISlave_total + IVreg_start = 9.6 + 3 = 12.6mA. It should probably be noted that the calculated current of 12.6mA is slightly below the 13mA maximum supply current that the LIN master is capable of supporting with a pull-up resistance of 220 ohms. In theory, says Rydahl, this network should be possible.
“The analysis and measurements here have shown that the existing LIN networking topology (three wires, battery, ground and LIN) can be easily transformed to a two-wire implementation (LIN and ground) with very little effort. All that is required is a thorough understanding of the system supply/load requirements and several hardware modifications to enable the slave node to harvest power from the master LIN bus line in between LIN data frame transmissions,” Rydahl added.
“The two-wire LIN network is best suited for low-node count networks where the system is limited to one master and no more than three slaves where all nodes are powered on simultaneously. The number of slave nodes could potentially be increased if the system designer is able to implement a power-on scheme where the slave nodes are activated serially to limit the surge current at network start-up.”