8.3.1 High Speed Forward Channel Data Transfer

The High Speed Forward Channel is composed of 35 bits of data containing RGB data, sync signals, HDCP, I2C, GPIOs, and I2S audio transmitted from serializer to deserializer. Figure 19 illustrates the serial stream per clock cycle. This data payload is optimized for signal transmission over an AC coupled link. Data is randomized, balanced and scrambled.

Figure 19. FPD-Link III Serial Stream

The DS90UH940-Q1 supports clocks in the range of 25 MHz to 96 MHz over a 1-lane, or 50MHz to 170MHz over 2-lanes. The FPD-Link III serial stream rate is 3.36 Gbps maximum (875 Mbps minimum) or 2.975 Gbps maximum per lane (875 Mbps minimum) respectively.

8.3.2 Low Speed Back Channel Data Transfer

The Low-Speed Backward Channel provides bidirectional communication between the display and host processor. The information is carried from the deserializer to the serializer as serial frames. The back channel control data is transferred over both serial links along with the high-speed forward data, DC balance coding and embedded clock information. This architecture provides a backward path across the serial link together with a high speed forward channel. The back channel contains the I2C, HDCP, CRC and 4 bits of standard GPIO information with 5 or 20 Mbps line rate (configured by MODE_SEL1).

8.3.3 FPD-Link III Port Register Access

Since the DS90UH940-Q1 contains two ports, some registers need to be duplicated to allow control and monitoring of the two ports. To facilitate this, PORT1_SEL and PORT0_SEL bits (0x34[1:0]) register controls access to the two sets of registers. Registers that are shared between ports (not duplicated) will be available independent of the settings in the PORT_SEL register.

Setting the PORT1_SEL and PORT0_SEL bit will allow a read of the register for the selected port. If both bits are set, port1 registers will be returned. Writes will occur to ports for which the select bit is set, allowing simultaneous writes to both ports if both select bits are set.

8.3.4 Clock and Output Status

When PDB is driven HIGH, the CDR PLL begins locking to the serial input and LOCK is tri-state or LOW (depending on the value of the OUTPUT ENABLE setting). After the deserializer completes its lock sequence to the input serial data, the LOCK output is driven HIGH, indicating valid data and clock recovered from the serial input is available on the LVCMOS and LVDS outputs. The State of the outputs is based on the OUTPUT ENABLE and OUTPUT SLEEP STATE SELECT register settings. See register 0x02 in Table 12.

8.3.5 LVCMOS VDDIO Option

The 1.8V or 3.3V Inputs and Outputs are powered from a separate VDDIO supply to offer compatibility with external system interface signals.

8.3.6 Power Down (PDB)

The deserializer has a PDB input pin to ENABLE or POWER DOWN the device. This pin can be controlled by the host or through the VDDIO, where VDDIO = 3. 0 V to 3.6 V or VDD33. To save power, disable the link when the display is not needed (PDB = LOW). When the pin is driven by the host, make sure to release it after VDD33 and VDDIO have reached final levels; no external components are required. In the case of driven by the VDDIO = 3.0 V to 3.6 V or VDD33 directly, a 10kΩ resistor to the VDDIO = 3.0 V to 3.6 V or VDD33, and a >10 µF capacitor to the GND are required (see Figure 37 Typical Connection Diagram).

8.3.7 Interrupt Pin — Functional Description and Usage (INTB_IN)

The INTB_IN pin is an active low interrupt input pin. This interrupt signal, when configured, will propagate to the paired serializer. Consult the appropriate Serializer datasheet for details of how to configure this interrupt functionality.

1. On the Serializer, set register 0xC6[5] = 1 and 0xC6[0] = 1
2. Deserializer INTB_IN (pin 4) is set LOW by some downstream device.
3. Serializer pulls INTB pin LOW. The signal is active LOW, so a LOW indicates an interrupt condition.
4. External controller detects INTB = LOW; to determine interrupt source, read HDCP_ISR register.
5. A read to HDCP_ISR will clear the interrupt at the Serializer, releasing INTB.
6. The external controller typically must then access the remote device to determine downstream interrupt source and clear the interrupt driving the Deserializer INTB_IN. This would be when the downstream device releases the INTB_IN (pin 4) on the Deserializer. The system is now ready to return to step (2) at next falling edge of INTB_IN.

8.3.8 General-purpose I/O

8.3.9 SPI Communication

The SPI Control Channel utilizes the secondary link in a 2-lane FPD-Link III implementation.  Two possible modes are available, Forward Channel and Reverse Channel modes.  In Forward Channel mode, the SPI Master is located at the Serializer, such that the direction of sending SPI data is in the same direction as the video data.  In Reverse Channel mode, the SPI Master is located at the Deserializer, such that the direction of sending SPI data is in the opposite direction as the video data.

The SPI Control Channel can operate in a high speed mode when writing data, but must operate at lower frequencies when reading data.  During SPI reads, data is clocked from the slave to the master on the SPI clock falling edge.  Thus, the SPI read must operate with a clock period that is greater than the round trip data latency.  On the other hand, for SPI writes, data can be sent at much higher frequencies where the MISO pin can be ignored by the master.

SPI data rates are not symmetrical for the two modes of operation.  Data over the forward channel can be sent much faster than data over the reverse channel.

Note: SPI cannot be used to access Serializer / Deserializer registers.

8.3.9.2 Forward Channel SPI Operation

In Forward Channel SPI operation, the SPI master located at the Serializer generates the SPI Clock (SPLK), Master Out / Slave In data (MOSI), and active low Slave Select (SS).  The Serializer oversamples the SPI signals directly using the video pixel clock.  The three sampled values for SPLK, MOSI, and SS are each sent on data bits in the forward channel frame.  At the Deserializer, the SPI signals are regenerated using the pixel clock.  In order to preserve setup and hold time, the Deserializer will hold MOSI data while the SPLK signal is high.  In addition, it delays SPLK by one pixel clock relative to the MOSI data, increasing setup by one pixel clock. 

Figure 20. Forward Channel SPI Write

Figure 21. Forward Channel SPI Read

8.3.9.3 Reverse Channel SPI Operation

In Reverse Channel SPI operation, the Deserializer samples the Slave Select (SS), SPI clock (SCLK) into the internal oscillator clock domain.  In addition, upon detection of the active SPI clock edge, the Deserializer samples the SPI data (MOSI).  The SPI data samples are stored in a buffer to be passed to the Serializer over the back channel.  The Deserializer sends SPI information in a back channel frame to the Serializer.  In each back channel frame, the Deserializer sends an indication of the Slave Select value. The Slave Select should be inactive (high) for at least one back-channel frame period to ensure propagation to the Serializer.

Because data is delivered in separate back channel frames and buffered, the data may be regenerated in bursts.  The following figure (Figure 22) shows an example of the SPI data regeneration when the data arrives in three back channel frames.  The first frame delivered the SS active indication, the second frame delivered the first three data bits, and the third frame delivers the additional data bits.

Figure 22. Reverse Channel SPI Write

For Reverse Channel SPI reads, the SPI master must wait for a round-trip response before generating the sampling edge of the SPI clock.  This is similar to operation in Forward channel mode.  Note that at most one data/clock sample will be sent per back channel frame.

Figure 23. Reverse Channel SPI Read

For both Reverse Channel SPI writes and reads, the SPI_SS signal should be deasserted for at least one back channel frame period.

Table 6. SPI SS Deassertion Requirement

Back Channel Frequency	Deassertion Requirement
5 Mbps	7.5 µs
10 Mbps	3.75 µs
20 Mbps	1.875 µs

8.3.10 Backward Compatibility

The DS90UH940-Q1 is also backward compatible to the DS90UH925Q-Q1, DS90UH925AQ-Q1, and DS90UH927Q-Q1 for PCLK frequencies ranging from 25MHz to 85MHz. Backward compatibility does not need to be enabled. When paired with a backward compatible device, the Deserializer will auto-detect to 1-lane FPD-Link III on the primary channel (RIN0±).

8.3.11 Input Equalization

An FPD-Link III input adaptive equalizer provides compensation for transmission medium losses and reduces medium-induced deterministic jitter. It equalizes up to 15m STP or 50Ω Coaxial cables with 3 connection breaks at maximum serializer stream payload of 3.36 Gbps.

8.3.12 I2S Audio Interface

This Deserializer features six I2S output pins that, when paired with a compatible serializer, supports surround sound audio applications. The bit clock (I2S_CLK) supports frequencies between 1MHz and the smaller of <PCLK/2 or <13MHz. Four I2S data outputs carry two channels of I2S-formatted digital audio each, with each channel delineated by the word select (I2C_WC) input.

Figure 24. I2S Connection Diagram

Figure 25. I2S Frame Timing Diagram

When paired with a DS90UH925Q , the Deserializer I2S interface supports a single I2S data output through I2S_DA (24-bit video mode), or two I2S data outputs through I2S_DA and I2S_DB (18-bit video mode).

8.3.13 HDCP

The HDCP Cipher function is implemented in the deserializer per HDCP v1.4 specification. The DS90UH940-Q1 provides HDCP decryption of audiovisual content when connected to an HDCP capable FPD-Link III serializer. HDCP authentication and shared key generation is performed using the HDCP Control Channel, which is embedded in the forward and backward channels of the serial link. On-chip Non-Volatile Memory (NVM) is used to store the HDCP keys. The confidential HDCP keys are loaded by TI during the manufacturing process and are not accessible external to the device.

8.3.14 Built-In Self Test (BIST)

An optional At-Speed Built-In Self Test (BIST) feature supports testing of the high speed serial link and the low-speed back channel without external data connections. This is useful in the prototype stage, equipment production, in-system test, and system diagnostics.

8.3.14.1 BIST Configuration And Status

The BIST mode is enabled at the deserializer by pin (BISTEN) or BIST configuration register. The test may select either an external PCLK or the 33 MHz internal Oscillator clock (OSC) frequency in the Serializer. In the absence of PCLK, the user can select the internal OSC frequency at the deserializer through the BISTC pin or BIST configuration register.

When BIST is activated at the deserializer, a BIST enable signal is sent to the serializer through the Back Channel. The serializer outputs a test pattern and drives the link at speed. The deserializer detects the test pattern and monitors it for errors. The deserializer PASS output pin toggles to flag each frame received containing one or more errors. The serializer also tracks errors indicated by the CRC fields in each back channel frame.

The BIST status can be monitored real time on the deserializer PASS pin, with each detected error resulting in a half pixel clock period toggled LOW. After BIST is deactivated, the result of the last test is held on the PASS output until reset (new BIST test or Power Down). A high on PASS indicates NO ERRORS were detected. A Low on PASS indicates one or more errors were detected. The duration of the test is controlled by the pulse width applied to the deserializer BISTEN pin. LOCK status is valid throughout the entire duration of BIST.

See Figure 26 for the BIST mode flow diagram.

8.3.14.1.1 Sample BIST Sequence

Note: Before BIST can be enabled, D_GPIO0 (pin 19) must be strapped HIGH and D_GPIO[3:1] (pins 16, 17, and 18) must be strapped LOW.

1. BIST Mode is enabled via the BISTEN pin of Deserializer. The desired clock source is selected through the deserializer BISTC pin.
2. The serializer is awakened through the back channel if it is not already on. An all-zeros pattern is balanced, scrambled, randomized, and sent through the FPD-Link III interface to the deserializer. Once the serializer and the deserializer are in BIST mode and the deserializer acquires LOCK, the PASS pin of the deserializer goes high and BIST starts checking the data stream. If an error in the payload (1 to 35) is detected, the PASS pin will switch low for one half of the clock period. During the BIST test, the PASS output can be monitored and counted to determine the payload error rate per 35 bits.
3. To Stop BIST mode, set the BISTEN pin LOW. The deserializer stops checking the data, and the final test result is held on the PASS pin. If the test ran error free, the PASS output will remain HIGH. If there one or more errors were detected, the PASS output will output constant LOW. The PASS output state is held until a new BIST is run, the device is RESET, or the device is powered down. BIST duration is user-controlled and may be of any length.

The link returns to normal operation after the deserializer BISTEN pin is low. Figure 27 shows the waveform diagram of a typical BIST test for two cases. Case 1 is error free, and Case 2 shows one with multiple errors. In most cases it is difficult to generate errors due to the robustness of the link (differential data transmission etc.), thus they may be introduced by greatly extending the cable length, faulting the interconnect medium, or reducing signal condition enhancements (Rx Equalization).

Figure 26. BIST Mode Flow Diagram

8.3.14.2 Forward Channel and Back Channel Error Checking

The Deserializer, on locking to the serial stream, compares the recovered serial stream with all-zeroes and records any errors in status registers. Errors are also dynamically reported on the PASS pin of the deserializer. Forward channel errors may also be read from register 0x25 (Table 12).

The back-channel data is checked for CRC errors once the serializer locks onto the back-channel serial stream, as indicated by link detect status (register bit 0x0C[0] - Table 12). CRC errors are recorded in an 8-bit register in the serializer. The register is cleared when the serializer enters the BIST mode. As soon as the serializer enters BIST mode, the functional mode CRC register starts recording any back channel CRC errors. The BIST mode CRC error register is active in BIST mode only and keeps the record of the last BIST run until cleared or the serializer enters BIST mode again.

Figure 27. BIST Waveforms

8.3.15 Internal Pattern Generation

The deserializer supports the internal pattern generation feature. It allows basic testing and debugging of an integrated panel. The test patterns are simple and repetitive and allow for a quick visual verification of panel operation. As long as the device is not in power down mode, the test pattern will be displayed even if no parallel input is applied. If no PCLK is received, the test pattern can be configured to use a programmed oscillator frequency. For detailed information, refer to Application Note AN-2198 (SNLA132).

8.4.1 Configuration Select

The DS90UH940-Q1 can be configured for several different operating modes via the MODE_SEL[1:0] input pins, or via the register bits 0x23 [4:3] (MODE_SEL1) and 0x6A [5:4] (MODE_SEL0) . A pull-up resistor and a pull-down resistor of suggested values may be used to set the voltage ratio of the MODE_SEL[1:0] input and VDD33 to select one of the possible selected modes.

The DS90UH940-Q1 is capable of operating in either in 1-lane or 2-lane modes for FPD-Link III. By default, the FPD-Link III receiver automatically configures the input based on 1- or 2-lane mode operation. Programming register 0x34 [4:3] settings will override the automatic detection. For each FPD-Link III pair, the serial datastream is composed of a 35-bit symbol.

The DS90UH940-Q1 recovers the FPD-Link III serial datastream(s) and produces CSI-2 TX data driven to the MIPI DPHY interface. There are two CSI-2 ports (CSI0_Dn and CSI1_Dn) and each consist of one clock lane and four data lanes. The DS90UH940-Q1 supports two CSI-2 TX ports, and each may be configured to support either two or four CSI-2 data lanes. Unused CSI-2 outputs are driven to LP11 states. The MIPI DPHY transmission operates in both differential (HS) and single-ended (LP) modes. During HS transmission, the pair of outputs operates in differential mode; and in LP mode, the pair operates as two independent single-ended traces. Both the data and clock lanes enter LP mode during the horizontal and vertical blanking periods.

The configurations outlined in (1-lane FPD-Link III Input, 4 MIPI lanes Output, 1-lane FPD-Link III Input, 2 MIPI lanes Output, 1- or 2-lane FPD-Link III Input, 2 or 4 MIPI lanes Output in Replicate) will apply to DS90UH949-Q1, DS90UH947-Q1, DS90UH929-Q1, DS90UH925Q-Q1, DS90UH925AQ-Q1, and DS90UH927Q-Q1 FPD-Link III Serializers.

The configurations outlined in (2-lane FPD-Link III Input, 4 MIPI lanes Output, 2-lane FPD-Link III Input, 2 MIPI lanes Output, 1- or 2-lane FPD-Link III Input, 2 or 4 MIPI lanes Output in Replicate) will apply to DS90UH949-Q1 and DS90UH947-Q1 FPD-Link III Serializers.

The device can be configured in following modes:

• 1-lane FPD-Link III Input, 4 MIPI lanes Output
• 1-lane FPD-Link III Input, 2 MIPI lanes Output
• 2-lane FPD-Link III Input, 4 MIPI lanes Output
• 2-lane FPD-Link III Input, 4 MIPI lanes Output
• 1- or 2-lane FPD-Link III Input, 2 or 4 MIPI lanes Output (Replicate)

8.4.2 MODE_SEL[1:0]

Configuration of the device may be done via the MODE_SEL[1:0] input pins, or via the configuration register bits. A pull-up resistor and a pull-down resistor of suggested values may be used to set the voltage ratio of the MODE_SEL[1:0] inputs (V_R4) and VDD33 to select one of the other 8 possible selected modes. See Table 8 and Table 9. Possible configurations are shown in Figure 28.

Figure 28. Datapath Configurations

Configuration of the device may be done via the MODE_SEL[1:0] input pins, or via the configuration register bits. A pull-up resistor and a pull-down resistor of suggested values may be used to set the voltage ratio of the MODE_SEL[1:0] inputs (V_R1) and VDD33 to select one of the other 8 possible selected modes. See Table 8 and Table 9.

Figure 29. MODE_SEL[1:0] Connection Diagram

8.4.3 CSI-2 Interface

The DS90UH940-Q1 (in default mode) takes RGB 24-bpp data bits defined in the serializer and directly maps to the pixel color space in the data frame. The DS90UH940-Q1 follows the general frame format as described per the CSI-2 standard (Figure 30). Upon the end of the vertical sync pulse (VS), the DS90UH940-Q1 generates the Frame End and Frame Start synchronization packets within the vertical blanking period. The timing of the Frame Start will not reflect the timing of the VS signal.

Upon the rising edge of the DE signal, each active line is output in a long data packet with the defined data format (Figure 13). At the end of each packet, the data lanes Dn± return to the LP-11 state, while the clock lane CLK± continue outputting the high speed clock.

The DS90UH940-Q1 CSI-2 transmitter consists of a high speed clock (CLK±) and data (Dn±) outputs based on a source synchronous interface. The half rate clock at CLK± is derived from the pixel clock sourced by the clock/data recovery circuit of the DS90UH940-Q1. The CSI-2 clock frequency is 3.5 times (4 MIPI lanes) or 7 times (2 MIPI lanes) the recovered pixel clock frequency. The MIPI DPHY outputs either 2 or 4 high speed data lanes (Dn±) according to the CSI-2 protocol. The data rate of each lane is 7 times (4 MIPI lanes) or 14 times (2 MIPI lanes) the pixel clock. As an example in a 4 MIPI lane configuration, at a pixel clock of 150 MHz, the CLK± runs at 525 MHz, and each data lane runs at 1050 Mbps.

The half-rate clock maintains a quadrature phase relationship to the data signals and allows receiver to sample data at the rising and falling edges of the clock (DDR). Figure 10 shows the timing relationship of the clock and data lines. The DS90UH940-Q1 supports continuous high speed clock. High speed data are sent out at data lanes Dn± in bursts. In between data bursts, the data lanes return to Low Power (LP) States in according to protocol defined in D-PHY standard. The rising edge of the differential clock (CSI_CLK+ – CSI_CLK-) is sent during the first payload bit of a transmission burst in the data lanes.  

Figure 30. CSI-2 General Frame Format

8.4.4 Input Display Timing

The DS90UH940-Q1 has built−in support to detect the incoming video format extracted from the FPD-Link III datastream(s) and automatically generate CSI-2 output timing parameters accordingly. The input video format detection is derived from progressive display resolutions based on the CEA−861D specification. The video data rate and frame rate is determined by measuring internal VS and DE signals.

8.4.5 MIPI CSI-2 Output Data Formats

The DS90UH940-Q1 CSI-2 Tx supports multiple data types. These can be seen in Table 10.

8.4.6 Non-Continuous / Continuous Clock

DS90UH940-Q1 D-PHY supports Continuous clock mode and Non-Continuous clock mode on the CSI-2 interface. Default mode is Non-Continuous Clock mode, where the Clock Lane enters in LP mode between the transmissions of data packets. Non-continuous clock mode will only be non-continuous during the vertical blanking period for lower PCLK rates. For higher PCLK rates, the clock will be non-continuous between line and frame packets. Operating modes are configurable through 0x6A [1].

Clock lane enters LP11 during horizontal blanking if the horizontal blanking period is longer than the overhead time to start/stop the clock lane. There is auto-detection of the length of the horizontal blank period. The fixed threshold is 96 PCLK cycles.

8.4.7 Ultra Low Power State (ULPS)

The DS90UH940-Q1 supports the MIPI defined Ultra-Low Power State (ULPS). DS90UH940-Q1 D-PHY lanes will enter ULPS mode upon software standby mode through 0x6A [2] generated by the processor. When ULPS is issued, all active CSI-2 lanes including the clock and data lanes of the enabled CSI-2 port are put in ULPS according to the MIPI DPHY protocol. D-PHY can reduce power consumption by entering ULPS mode. Ultra Low Power State is exited by means of a Mark-1 state with a length TWAKEUP followed by a Stop state.

Figure 31. Ultra Low Power State

8.4.8 CSI-2 Data Identifier

The DS90UH940-Q1 MIPI CSI-2 protocol interface transmits the data identifier byte containing the values for the virtual channel ID (VC) and data type (DT) for the application specific payload data, as shown in Figure 32. The virtual channel ID is contained in the 2 MSBs of the data identifier byte and identify the data as directed to one of four virtual channels. The value of the data type is contained in the 6 LSBs of the data identifier byte.

• CSIIA_{0x6C}_0x2E[7:6] CSI_VC_ID: Configures the virtual ID linked to the current context.
• CSICFG1_0x6B[7:4] OFMT: Configures the data format linked to the current context.

Figure 32. CSI-2 Data Identifier Structure

8.5.1 Serial Control Bus

The device may also be configured by the use of a I2C compatible serial control bus. Multiple devices may share the serial control bus (up to 8 device addresses supported). The device address is set via a resistor divider (R1 and R2 — see Figure 33 below) connected to the IDx pin.

Figure 33. Serial Control Bus Connection

The serial control bus consists of two signals, SCL and SDA. SCL is a Serial Bus Clock Input. SDA is the Serial Bus Data Input / Output signal. Both SCL and SDA signals require an external pull-up resistor to 1.8 V or 3.3 V VDDIO. For most applications, a 4.7kΩ pull-up resistor to VDD33 is recommended. However, the pull-up resistor value may be adjusted for capacitive loading and data rate requirements. The signals are either pulled High, or driven Low.

The IDx pin configures the control interface to one of 8 possible device addresses. A pull-up resistor and a pull-down resistor may be used to set the appropriate voltage ratio between the IDx input pin (V_R2) and VDD33, each ratio corresponding to a specific device address. See Table 11 below.

The Serial Bus protocol is controlled by START, START-Repeated, and STOP phases. A START occurs when SCL transitions Low while SDA is High. A STOP occurs when SDA transitions High while SCL is also HIGH. See Figure 34

Figure 34. START and STOP Conditions

To communicate with a remote device, the host controller (master) sends the slave address and listens for a response from the slave. This response is referred to as an acknowledge bit (ACK). If a slave on the bus is addressed correctly, it Acknowledges (ACKs) the master by driving the SDA bus low. If the address doesn't match a device's slave address, it Not-acknowledges (NACKs) the master by letting SDA be pulled High. ACKs also occur on the bus when data is being transmitted. When the master is writing data, the slave ACKs after every data byte is successfully received. When the master is reading data, the master ACKs after every data byte is received to let the slave know it wants to receive another data byte. When the master wants to stop reading, it NACKs after the last data byte and creates a stop condition on the bus. All communication on the bus begins with either a Start condition or a Repeated Start condition. All communication on the bus ends with a Stop condition. A READ is shown in Figure 35 and a WRITE is shown in Figure 36.

Figure 35. Serial Control Bus — READ

Figure 36. Serial Control Bus — WRITE

The I2C Master located at the Deserializer must support I2C clock stretching. For more information on I2C interface requirements and throughput considerations, please refer to TI Application Note SNLA131.

8.5.2 Multi-Master Arbitration Support

The Bidirectional Control Channel in the FPD-Link III devices implements I2C compatible bus arbitration in the proxy I2C master implementation. When sending a data bit, each I2C master senses the value on the SDA line. If the master is sending a logic 1 but senses a logic 0, the master has lost arbitration. It will stop driving SDA, retrying the transaction when the bus becomes idle. Thus, multiple I2C masters may be implemented in the system.

For example, there might also be a local I2C master at each camera. The local I2C master could access the Image Sensor and EEPROM. The only restriction would be that the remote I2C master at the camera should not attempt to access a remote slave through the BCC that is located at the host controller side of the link. In other words, the control channel should only operate in camera mode for accessing remote slave devices to avoid issues with arbitration across the link. The remote I2C master should also not attempt to access the deserializer registers to avoid a conflict in register access with the Host controller.

If the system does require master-slave operation in both directions across the BCC, some method of communication must be used to ensure only one direction of operation occurs at any time. The communication method could include using available read/write registers in the deserializer to allow masters to communicate with each other to pass control between the two masters. An example would be to use register 0x18 or 0x19 in the deserializer as a mailbox register to pass control of the channel from one master to another.

8.5.3 I2C Restrictions on Multi-Master Operation

The I2C specification does not provide for arbitration between masters under certain conditions. The system should make sure the following conditions cannot occur to prevent undefined conditions on the I2C bus:

• One master generates a repeated Start while another master is sending a data bit.
• One master generates a Stop while another master is sending a data bit.
• One master generates a repeated Start while another master sends a Stop.

Note that these restrictions mainly apply to accessing the same register offsets within a specific I2C slave.

8.5.4 Multi-Master Access to Device Registers for Newer FPD-Link III Devices

When using the latest generation of FPD-Link III devices (DS90UH94x-Q1), serializers or deserializer registers may be accessed simultaneously from both local and remote I2C masters. These devices have internal logic to properly arbitrate between sources to allow proper read and write access without risk of corruption.

Access to remote I2C slaves would still be allowed in only one direction at a time (Camera or Display mode).

8.5.5 Multi-Master Access to Device Registers for Older FPD-Link III Devices

When using older FPD-Link III devices (in backward compatible), simultaneous access to serializer or deserializer registers from both local and remote I2C masters may cause incorrect operation, thus restrictions should be imposed on accessing of serializer and deserializer registers. The likelihood of an error occurrence is relatively small, but it is possible for collision on reads and writes to occur, resulting in an errored read or write.

Two basic options are recommended. The first is to allow device register access only from one controller. In a Display mode system, this would allow only the Host controller to access the serializer registers (local) and the deserializer registers (remote). A controller at the deserializer (local to the Display) would not be allowed to access the deserializer or serializer registers.

The second basic option is to allow local register access only with no access to remote serializer or deserializer registers. The Host controller would be allowed to access the serializer registers while a controller at the deserializer could access those register only. Access to remote I2C slaves would still be allowed in one direction (Camera or Display mode).

In a very limited case, remote and local access could be allowed to the deserializer registers at the same time. Register access is ensured to work correctly if both local and remote masters are accessing the same deserializer register. This allows a simple method of passing control of the Bidirectional Control Channel from one master to another.

8.5.6 Restrictions on Control Channel Direction for Multi-Master Operation

Only Display or Camera mode operation should be active at any time across the Bidirectional Control Channel. If both directions are required, some method of transferring control between I2C masters should be implemented.

Table 12. Serial Control Bus Registers

Table 13. CSI Indirect Register Table