Application of Dual Automotive Electronic Control Units Based on GMSL SerDes

Maxim's gigabit multimedia serial link (GMSL) solution serializes digital video and audio data and transmits it serially over a twisted pair of wires. In addition, an integrated bidirectional control channel enables a single microprocessor (µC) to program the serializer, deserializer, and all connected peripherals.

In typical applications, the remote microprocessor and related components, such as clock source/crystal and low-voltage power supply, can be omitted. This solution not only simplifies the remote design, but also reduces system cost, size and power consumption.

However, in some cases, a µC is still present at both ends of the link due to special requirements beyond GMSL. This application note describes how to connect two µCs to control GMSL.

Dual µC Application Basics

When using a single µC, the control direction select pin (CDS) on both ends of the serializer/deserializer is usually set low if the µC is on the serializer side, and high if the µC is on the deserializer side. However, if the serializer CDS is set low and the deserializer CDS is set high, each GMSL chip can be connected to its own corresponding µC at the same time (Figure 1).

Figure 1. Simple dual-µC application schematic with CDS setup shown.

Internal Operations

When using two µCs, the I²C master for both the serializer and deserializer is disabled, and the RX/SDA and TX/SDL are configured as UART interfaces by their respective µCs. Since each device operates as a local device, it cannot go into sleep mode.

Figure 2. Serializer state diagram (CDS = low)

Figure 3. Deserializer state diagram (CDS = high)

Each device is driven into a low-power state using the corresponding active-low PWDN pin. Remember that all device settings are reset to their power-on initial values ​​when waking up from a power-down state.

Conflicts in Dual µC Applications

In the configuration shown in Figure 1, each µC can communicate with the MAX9259 serializer, MAX9260 deserializer, or other µCs using the GMSL UART protocol. GMSL does not provide collision avoidance measures, so the user must provide their own collision handling measures.

Independent Networking

The simplest way to avoid collisions is to have each µC set the FWDCCEN and REVCCEN bits of its attached serializer/deserializer to 0 (0x04 D[1:0]). This approach disables the receiver and transmitter of the forward and reverse control channels and effectively splits the control network into two independent networks (Figure 4).

Figure 4. Independent control networks avoid the possibility of conflicts.

Any communication over the serial link first requires the µC on each side to re-enable communications on the respective end of the link. This setup works well in “always-on” applications where the settings of the critical link-specific registers are not changed from their initial state.

Software conflict resolution

In applications where communication between the two ends of a serial link is necessary, users can avoid conflicts by using higher-level protocols (Figure 5). In the following example, each µC waits for an ACK frame to determine if its command was successful.

Figure 5. Example of software handling conflicts

When a collision occurs, the serializer/deserializer does not send out an ACK frame. After failing to receive an ACK frame, the µC waits for a period of time based on their device address before resending the command. Since the microprocessors in this design have different device addresses, there will be no collision when retrying communication.

Single/Dual µC Applications

Some applications do not require both µCs to remain operational at all times. If the CDS input on either end changes state during operation, the corresponding device resumes operation following the link startup procedure described in the MAX9259 data sheet.

Switching between single and dual µC operation as needed, enabling GMSL in turn will take up fewer resources. Unused µCs can be shut down to reduce power consumption, helping to extend battery life.

Remote display example (deserializer)

In the following application, the deserializer side of the link is a display panel configured for remote power on/off. The board shutdown input and single/dual µC control are connected to the output of MAX9260 GPIO0 (Figure 6).

Figure 6. Single/dual µC remote display example

Once powered up, the GPIO outputs high to keep the remote device off, and the deserializer is configured as a remote device due to the additional inverter. Since the MS is connected to the GPIO, the MAX9260 is powered up in sleep mode, and all other devices are in low-power states.

To turn on the remote panel, the serializer wakes up the MAX9260 and establishes the serial link. The µC on the serializer side then sets GPIO0 low, which drives MS low and the inverter output high. The inverter sets the MAX9260 as a local device and wakes up the rest of the remote display panel circuitry. MS must be driven low to maintain the basic mode of the MAX9260 UART interface.

To shut down the remote panel, the serializer sets GPIO0 high to shut down the remote device and sets the MAX9260 as the remote device. Then, it sets SLEEP = 1 in the MAX9260 to put the device into sleep mode.

Remote Camera Example (Serializer)

Similar to the previous example, the serializer side of the link is a camera module configured for remote power on/off. The MAX9259's INT output controls the board's shutdown input and single/dual µC switching (Figure 7).

Figure 7. Single/dual µC remote camera example

In this application, INT is used as a GPO, and the output is controlled by setting SETINT (0x0D D7 of the MAX9259) or the INT input of the deserializer. Upon power-up, the INT output is low, keeping the remote device powered off. The inverter output is connected to CDS, configuring the serializer as a remote device. The MAX9259 powers up in sleep mode because the active-low AUTOS is set high.

To power up the remote panel, the deserializer wakes up the MAX9259 with a GMSL UART command. The deserializer then sets the MAX9259's INT output high, powering up all remote devices. The inverter output sets the MAX9259 as a local device, ready to receive UART commands from the local µC.

To power down the remote panel, the deserializer sets the MAX9259's INT output low, which powers down the remote device and sets the MAX9259 as the remote device. The deserializer then sets SLEEP = 1 in the MAX9259, which puts the device into sleep mode.

Other applications

Dual µC applications are not limited to the examples above. Symmetrical, bidirectional control panels, along with real-time CDS and bypass settings (via MS) can enable numerous SerDes and µC configurations. Designers need greater control to increase system capabilities and minimize system power consumption, maximizing the use of existing resources.