Theory of Operation

Block Diagram

Design Details

Functional Modes

I2C IP is a host-target combo that can function as either an I2C host or an I2C target. Although it is conceivable that an I2C combo can optionally function as both a host and a target at the same time, we do not support this feature at this time. These functional modes are enabled at runtime by setting the register fields CTRL.ENABLEHOST and CTRL.ENABLETARGET.

Virtual Open Drain

In devices which lack a true open drain buffer functionality, this IP implements a “virtual Open Drain” functionality. The SDA and SCL outputs are assumed to be connected to a tri-state buffer, with independent enable outputs for both signals.

Rather than toggling the buffer inputs, the buffer inputs are continuously asserted low, and instead the buffer enable signals are toggled. The SDA or SCL buffers are enabled for a logical “Low” output on the respective signal, and are disabled for logical “High” outputs. This arrangement allows the output pins to float high if there is no conflict from external devices, or to be pulled low if there is a conflict (as is required for clock-stretching or–in future revisions– multi-host functionality).

This arrangement is necessary for FPGA builds.

Override Mode for Direct Pin Access

The I2C hardware interface consists of two external pins, SCL and SDA, whose behavior is described in the I2C specification (rev. 6). These pins are typically controlled by an internal state machine. However, there is a simpler “override” mode, by which these pins can be directly manipulated by software. This override mode is useful for both troubleshooting and error recovery.

To enter override mode, the register field OVRD.TXOVRDEN is asserted by software. In this state the output drivers scl_tx_o and sda_tx_o are controlled directly by the register fields OVRD.SCLVAL and OVRD.SDAVAL. When OVRD.SCLVAL and OVRD.SDAVAL are set high, the virtual open drain configuration will leave the output resistively pulled high, and controllable by remote targets. In this state, with SCL or SDA asserted high, the register fields VAL.SCL_RX and VAL.SDA_RX can be used to receive inputs (including remote acknowledgments) from target devices.

FSM control of SCL and SDA

While in host mode, SCL and SDA are generated through the internal state machine. Since SCL is directly decoded from the states, it can have short glitches during transition which the external target may be sensitive to if it is not using an over-sampling scheme. To counter this, the SCL and SDA outputs from the internal state machine are flopped before they are emitted.

This adds a one cycle module clock delay to both signals. If the module clock is sufficiently faster than I2C line speeds (for example 20MHz), this is not an issue. However if the line speeds and the module clock speeds become very close (2x), the 1 cycle delay may have an impact, as the internal state machine may mistakenly think it has sampled an SDA that has not yet been updated.

Therefore, it is recommended that the internal module clock frequency is much higher than the line speeds. Another reason to have this higher internal clock frequency is that the timing parameters can be more accurately defined, which helps attain the desired I2C clock rate. Since there are currently also a few cycles discrepancy between the specified timings and the actual ones (as described in the Programmer’s Guide), it is recommended that the internal module clock frequency is at leat 50x higher than the I2C line speeds.

Byte-Formatted Programming Mode

This section applies to I2C in the host mode. The state-machine-controlled mode allows for higher-speed operation with less frequent software interaction. In this mode, the I2C pins are controlled by the state machine, which in turn is controlled by a sequence of formatting indicators. These indicators determine:

  • The sequence of bytes which should be transmitted on the SDA and SCL pins.
  • The periods between transmitted bytes when the state-machine should stop transmission and instead read back a fixed number of bytes.
  • Which bytes should be preceded by a START symbol.
  • Which bytes should be followed by a STOP symbol.

The format indicator consists of 13-bits. That is of one single format byte (entered into the format FIFO through FDATA.FBYTE), and five 1-bit flags (entered into the format FIFO through registers FDATA.READB, FDATA.RCONT, FDATA.START, FDATA.STOP and FDATA.NAKOK)

The I2C reads each format indicator from the head of FMT_FIFO, and processes them in turn. If none of the flags are set for the format indicator, the I2C FSM simply transmits the format byte onto the SCL and SDA pins according to the specification, waits for acknowledgement, and then proceeds to the next format indicator. The format flags modulate the behavior as follows.

  • READB (corresponds to FDATA.READB): Signifies the format byte (FDATA.FBYTE) should be treated as an unsigned number, R, and prompts the state machine to read R bytes from the target device. Bytes read from the bus are inserted into the RX FIFO where they can be accessed by software. A value of 0 is treated as a read of 256 bytes. To read a larger byte stream, multiple 256-byte reads can be chained together using the RCONT flag.
  • RCONT (corresponds to FIFO inputs FDATA.RCONT, only used with READB):
    • If RCONT is set, the format byte represents part of a longer sequence of reads, allowing for reads to be chained indefinitely.
    • The RCONT flag indicates the final byte returned with the current read should be responded to with an ACK, allowing the target to continue sending data. (Note that the first R-1 bytes read will still be acknowledged regardless of whether RCONT is asserted or not.)
  • START (corresponds to FDATA.START, Ignored when used with READB): Issue a START condition before transmitting the format byte on the bus. This flag may also be used to issue a repeated start condition.
  • STOP (corresponds to FDATA.STOP): Issue a STOP signal after processing this current entry in the FMT FIFO. Note that this flag is not compatible with (READB & RCONT), and will cause bus conflicts.
  • NAKOK (corresponds to FDATA.NAKOK, Not compatible with READB): Typically every byte transmitted must also receive an ACK signal, and the IP will raise an exception if no ACK is received. However, there are some I2C commands which do not require an ACK. In those cases this flag should be asserted with FBYTE indicating no ACK is expected and no interrupt should be raised if the ACK is not received.


If the Host-Mode Controller-Transmitter transmits a byte and the 9th bit is a NACK from the Target/Bus, the controller_halt interrupt is usually asserted (modulo the effect of FDATA.NAKOK). If the controller_halt interrupt is asserted, the Byte-Formatted Programming Mode FSM will halt until the interrupt condition has been cleared in the CONTROLLER_EVENTS.NACK register. Software will likely want to do one of two things in handling this irq:

  1. End the current transaction by setting CTRL.ENABLEHOST to 1'b0, and clear the FIFOs to be ready to begin a new transaction.
  2. Clear the FIFOs, then begin a new transfer without ending the transaction by adding new FDATA indicators to the FMTFIFO. In this case, the first entry written to the FMTFIFO should have FDATA.START set to 1'b1 to create a repeated start condition.

Software should then clear the CONTROLLER_EVENTS.NACK to proceed with the next operation. The controller_halt interrupt is a CIP status type, so it doesn’t clear by writing to the INTR_STATE.CONTROLLER_HALT bit. Instead, the CONTROLLER_EVENTS CSR tracks the contributing events, and all must be cleared for the interrupt to de-assert.

The Halt-on-‘NAK’ behaviour may be problematic if SW is not responsive, so there is a timeout mechanism that can automatically end the transaction by creating a STOP condition and returning to halt in an idle state. This is configured using the HOST_NACK_HANDLER_TIMEOUT CSR, which allows software to configure the delay before hardware will terminate the transaction. When the timeout occurs, the [CONTROLLER_EVENTS.UNHANDLED_NACK_TIMEOUT] bit will set, and this condition will also cause the controller_halt interrupt to assert (or remain asserted). Like the original NACK event, the timeout bit will cause the controller to remain halted until it is cleared. To continue with a new transaction, software should ensure that the controller_halt interrupt has been handled, with all contributing event bits cleared.

When the FSM first halts due to an unexpected NACK, there is still an ongoing transaction. The controller stops immediately following the (N)ACK bit’s hold time after the SCL falling edge, and SCL remains asserted low. If a NACK handling timeout occurs or [CTRL.ENABLEHOST] is cleared, then the FSM will halt again after the STOP condition is sent. The STOP condition terminates the transaction, leaving the FSM halted in the “bus idle” state, with SDA and SCL released.

Target Address Registers

I2C target device is assigned two 7-bit address and 7-bit mask pairs. The target device accepts a transaction if the result of the bitwise AND operation performed on the transaction address sent by the host and a mask matches the assigned address corresponding to the mask. In other words, address matching is performed only for bits where the mask is “1”. Thus, with the masks set to all ones (0x7F), the target device will respond to either of the two assigned unique addresses and no other. If the mask and the assigned address both have zeros in a particular bit position, that bit will be a match regardless of the value of that bit received from the host. Note that if, in any bit position, the mask has zero and the assigned address has one, no transaction can match and such mask/address pair is effectively disabled.

Finally, in the special case where the mask is all zeroes (0x00), then that corresponding entry is ignored and will not match. If the user wishes to have the target respond to all addresses, then this can be achieved by setting mask0 and mask1 to 0x01 and setting address0 to 0x00 and address1 to 0x01.

The assigned address and mask pairs are set in registers TARGET_ID.ADDRESS0, TARGET_ID.MASK0, TARGET_ID.ADDRESS1, and TARGET_ID.MASK1.

Acquired Formatted Data

This section applies to I2C in the target mode. When the target accepts a transfer, it inserts the transfer address, read/write bit, and START signal sent by the host into ACQ FIFO where they can be accessed by software. ACQ FIFO output corresponds to ACQDATA. If the transfer is a write operation (R/W bit = 0), the target proceeds to read bytes from the bus and insert them into ACQ FIFO until the host terminates the transfer by sending a STOP or a repeated START signal. A STOP or repeated START indicator is inserted into ACQ FIFO as the next entry following the last byte received, in which case other bits may be junk or the target address, respectively.

Note that the repeated START indicator only appears if it matches the address. Typically, this is the case, and a transaction that changes targets between transfers is strongly discouraged. If no additional transfers targeting this target’s address occur, an entry representing the end of the transfer will wait for the STOP symbol.

The following diagram shows consecutive entries inserted into ACQ FIFO during a write operation:

If the transaction is a read operation (R/W bit = 1), the target pulls bytes out of TX FIFO and transmits them to the bus until the host signals the end of the transfer by sending a NACK signal. If TX FIFO holds no data, or if the ACQ FIFO contains more than 1 entry, the target will hold SCL low to stretch the clock and give software time to write data bytes into TX FIFO or handle the available command. See stretching during read for more details. TX FIFO input corresponds to TXDATA. Typically, a NACK signal is followed by a STOP or repeated START signal and the IP will raise an exception if the host sends a STOP signal after an ACK. An ACK/NACK signal is inserted into the ACQ FIFO as the first bit (bit 0), in the same entry with a STOP or repeated START signal. For ACK and NACK signals, the value of the first bit is 0 and 1, respectively. The following diagram shows consecutive entries inserted into ACQ FIFO during a read operation:

The ACQ FIFO entry consists of 11 bits:

  • Address (bits 7:1) and R/W bit (bit 0) or data byte
  • Format flags (bits 10:8) The format flags indicate the following signals received from the host:
  • START: 001
  • STOP completing transaction without errors: 010
  • repeated START: 011
  • No START, or STOP: 000
  • NACK during target-receiver transfer: 100
  • NACK of this target’s address: 101
  • STOP received for a transaction with errors: 110

Timing Control Registers

For Standard-mode, Fast-mode and Fast-mode Plus, the timing requirements for each transaction are detailed in Table 10 of the I2C specification (rev. 6). To claim complete compatibility at each mode, the state machine timings need to be adapted to whether there are Standard-mode, Fast-mode and Fast-mode Plus targets on the bus. Furthermore, depending on the actual capacitance of the bus, even a bus with all Fast-mode Plus capable targets may have to operate at slower speeds than 1 Mbaud. For example, the host may need to run at lower frequencies, as discussed in Section 5.2 of the specification, but the computation of the nominal frequency will depend on timing specifications in Table 10, in this case particularly, the limits on tLOW, tHIGH, tr, and tf. Assuming no clock stretching, for a given set of these four parameters the baud rate is then given to be: $$ 1/f_{SCL}=t_{LOW}+t_{HIGH}+t_{r}+t_{f}. $$

Thus in order to ensure compliance with the spec in any particular configuration, software will program the I2C host IP with explicit values for each of the following timing parameters, as defined in Figure 38 of the specification.

  • tLOW: set in register TIMING0.TLOW.
  • tHIGH: set in register TIMING0.THIGH.
  • tr: set in register TIMING1.T_R. (Note: The rise time cannot be explicitly controlled by internal hardware, and will be a function of the capacitance of the bus. Thus this parameter is largely budgetary, meaning that it tells the state machine how much time to wait for an RC rise.)
  • tf: set in register TIMING1.T_F. (Note: The fall time cannot be explicitly controlled by internal hardware, and is a function of the pin driver. Thus this parameter is also budgetary. Given that the actual fall time cannot be controlled to stay above the minimum values set in Table 10 of the specification, and so this in this regard this module currently is not strictly compliant to the I2C spec.)
  • tSU,STA: set in register TIMING2.TSU_STA
  • tHD,STA: set in register TIMING2.THD_STA
  • tSU,DAT: set in register TIMING3.TSU_DAT. Taken to be synonymous with TSU,ACK
  • tHD,DAT: set in register TIMING3.THD_DAT. Taken to be synonymous with THD,ACK. Moreover, since the pin driver fall time is likely to be less then one clock cycle, this parameter is also taken to be synonymous with the parameters TVD,DAT and TVD,ACK This parameter controls the number of cycles after the falling edge of SCL that SDA is driven. In addition, when the IP operates as a target, the parameter specifies the required SCL high time after SDA changes to satisfy the Start and Stop symbol decoders.
  • tSU,STO: set in register TIMING4.TSU_STO.
  • tBUF: set in register TIMING4.T_BUF

The values programmed into the registers TIMING0 through TIMING4 are to be expressed in units of the input clock period. It is important that the internal clock is at least 50x the bus clock so that the proportions of the timings can be accurately captured. Note in order to ensure compliance with the I2C spec, firmware must program these registers with values within the ranges laid out in Table 10 of the specification. These values can be directly computed using DIFs given the desired speed standard, the desired operating frequency, and the actual line capacitance. These timing parameters are then fed directly to the I2C state machine to control the bus timing.

A detailed description of the algorithm for determining these parameters–as well as a couple of concrete examples–are given in the Programmer’s Guide.

Timeout Control

A malfunctioning (or otherwise very slow) target device can hold SCL low indefinitely, stalling the bus. For this reason TIMEOUT_CTRL provides a clock-stretching timeout mechanism to notify firmware of this sort of condition. If TIMEOUT_CTRL.EN is asserted, an interrupt will be asserted when the IP detects that another device (a target or, in possible future revisions, an alternate host) has been holding SCL low for more than TIMEOUT_CTRL.VAL clock ticks.

This feature is added as a utility, though it is not required by the I2C specification. However, in some applications it could be used in protocols which build upon I2C. For instance, SMBus applications using this IP could in principle use this to support SMBus timeouts. (Note: This is just an example application of this feature. Other features may also be required for complete SMBus functionality.)

Clock Stretching

As described in the I2C specification, a target device can pause a transaction by holding SCL low. There are 3 cases in which this design stretches the clock. In all cases described below, a target begins to stretch the clock after the ACK bit. In the first two scenarios, it is after the ACK bit sent by the target, in the last scenario, it is after the host’s ACK bit.

Stretching after address read

  • When a target device receives a start, the address and R/W bit are written into the ACQ FIFO.
  • If there is no space in the ACQ FIFO to receive such a write, the target stretches the clock after the ACK bit and waits for software to make space.
  • The acq_stretch interrupt is generated to alert software to such a situation.

Stretching during write

  • Similar to the scenario above, if the host tries to write a data byte into the ACQ FIFO when there is no available space, the clock is also stretched after the ACK bit.
  • The acq_stretch interrupt is generated to alert software to such a situation.

Stretching during read

  • When a target device receives a start and read command, it may stretch the clock for either of the following two reasons.
    • If there is no data available to be sent back (TX FIFO empty case), the target stretches the clock until data is made available by software.
    • If there is more than 1 entry in the ACQ FIFO.
      • Having more than 1 entry in the ACQ FIFO suggests there is potentially an unhandled condition (STOP / RESTART) or an unhandled command (START) that requires software intervention before the read can proceed.
  • The tx_stretch interrupt is generated to alert software to such a situation.

Target ACK Control Mode

ACK Control Mode is a feature that allows software to take control of ACK/NACK decisions during a Write transfer. When ACK Control Mode is disabled in CTRL.ACK_CTRL_EN, the Target module will automatically ACK any incoming data byte, so long as there is sufficient space in the ACQ FIFO. If there isn’t enough space for the data byte and a STOP, the Target module will stretch the clock until space is made available (or a stretch timeout occurs). However, if ACK Control Mode is enabled, the TARGET_ACK_CTRL CSR can also cause the Target module to stretch the clock at the (N)ACK phase.

In ACK Control Mode, an Auto ACK Counter influences whether the Target module automatically ACKs a data byte when there is space in the ACQ FIFO. The counter begins at 0 for every transfer, and it decrements for every data byte that is ACK’d. In addition, this counter only applies to the bytes in a Write transfer that follow the address byte. When a transfer reaches the (N)ACK phase and the TARGET_ACK_CTRL.NBYTES CSR is 0, the Target module stretches the clock and awaits instructions from software, up to the stretch timeout. Note that because the Auto ACK Counter begins every transfer at 0, the first data byte will always stretch the clock.

To release SCL before the timeout, software can choose whether to ACK or NACK the byte. To ACK the byte, software would program the Auto ACK Counter to a greater number, and if the value programmed is greater than 1, it would automatically ACK subsequent bytes as well. Even with a nonzero count, though, the Target module could still stop and stretch the clock if the ACQ FIFO doesn’t have sufficient space. Otherwise, software can explicitly NACK that byte and the rest of the transaction by writing a 1 to TARGET_ACK_CTRL.NACK. If the Target module is not stretching the clock, writes to TARGET_ACK_CTRL are ignored.

With the data byte pending in the (N)ACK phase, it hasn’t been written to the ACQ FIFO yet. Instead, software can read the ACQ_FIFO_NEXT_DATA CSR to decide what to do. Note that after software indicates its decision, this pending byte will still enter the ACQ FIFO if there is space, accompanied by the metadata reflecting whether it was ACK’d or NACK’d.

This feature is intended to support protocols that require mid-transfer NACK decisions based on the current data transferred (e.g. as in SMBus).


The I2C module has a few interrupts including general data flow interrupts and unexpected event interrupts.

Host Mode

Whenever the RX FIFO exceeds the designated number of entries, the interrupt rx_threshold is asserted to inform firmware. Firmware can configure the threshold value via the register HOST_FIFO_CONFIG.RX_THRESH.

Whilst the FMT FIFO level lies below a designated number of entries, the fmt_threshold interrupt is asserted. Firmware can configure the threshold value via the register HOST_FIFO_CONFIG.FMT_THRESH.

If the RX FIFO receives an additional write request when its FIFO is full, the interrupt rx_overflow is asserted and the character is dropped.

If the module transmits a byte and the 9th bit is a NACK from the Target/Bus, the nak interrupt is usually asserted (modulo the effect of FDATA.NAKOK). If the ‘nak’ interrupt is asserted, the Byte-Formatted Programming Mode FSM will halt until the interrupt has been acknowledged. See the Halt-on-NAK section above for more details on this behaviour.

When the I2C module is in transmit mode, the scl_interference or sda_interference interrupts will be asserted if the IP identifies that some other device (host or target) on the bus is forcing either signal low and interfering with the transmission. It should be noted that the scl_interference interrupt is not raised in the case when the target device is stretching the clock. (However, it may be raised if the target allows SCL to go high and then pulls SCL down before the end of the current clock cycle.)

A target device should never assert 0 on the SDA lines, and in the absence of multi-host support, the sda_interference interrupt is raised whenever the host IP detects that another device is pulling SDA low.

On the other hand, it is legal for the a target device to assert SCL low for clock stretching purposes. With clock stretching, the target can delay the start of the following SCL pulse by holding SCL low between clock pulses. However the target device must assert SCL low before the start of the SCL pulse. If SCL is pulled low during an SCL pulse which has already started, this interruption of the SCL pulse will be registered as an exception by the I2C core, which will then assert the scl_interference interrupt.

{signal: [
  {name: 'Clock', wave: 'p.....|.......|......'},
  {name: 'SCL Host Driver', wave: '0.z..0|.z....0|..z.x.'},
  {name: 'SCL Target Driver', wave: 'z.....|0..z...|...0..'},
  {name: 'SCL bus', wave: '0.u..0|...u..0|..u0..'},
  {name: 'scl_interference', wave: '0.....|.......|....1.'},
  head: {text: 'SCL pulses: Normal SCL pulse (Cycle 3),  SCL pulse with clock stretching (cycle 11), and SCL interference (interrupted SCL pulse)',tick:1}}

Though normal clock stretching does not count as SCL interference, if the module detects that a target device has held SCL low and stretched the any given SCL cycle for more than TIMEOUT_CTRL.VAL clock ticks this will cause the stretch timeout interrupt to be asserted. This interrupt is suppressed, however, if TIMEOUT_CTRL.EN is deasserted low.

{signal: [
  {name: 'Clock', wave: 'p............'},
  {name: 'SCL Host Driver', wave: '0..z.......x.'},
  {name: 'SCL Target Driver', wave: 'z0...........'},
  {name: 'SCL bus', wave: '0............'},
  {name: 'TIMEOUT_CNTRL.VAL', wave: '2............', data: "8"},
  {name: 'SCL timeout counter', wave: '2...22222222x', data: '0 1 2 3 4 5 6 7 8'},
  {name: 'TIMEOUT_CNTRL.EN', wave: '1............'},
  {name: 'scl_timeout', wave: '0..........1.'},
  head: {text: 'SCL Timeout Example',tick:-3}}

Except for START and STOP symbols, the I2C specification requires that the SDA signal remains constant whenever SCL is high. The sda_unstable interrupt is asserted if, when receiving data or acknowledgement pulse, the value of the SDA signal does not remain constant over the duration of the SCL pulse.

Transactions are terminated by a STOP signal. The host may send a repeated START signal instead of a STOP, which also terminates the preceding transaction. In both cases, the cmd_complete interrupt is asserted, in the beginning of a repeated START or at the end of a STOP.

Target Mode

The interrupt cmd_complete is asserted whenever a RESTART or a STOP bit is observed by the target.

The interrupt tx_stretch is asserted whenever target intends to transmit data but cannot. See [stretching during read]({{< relref “#stretching-during-read” >}}).

When a host receives enough data from a target, it usually signals the end of the transaction by sending a NACK followed by a STOP or a repeated START. In a case when a target receives a STOP without the prerequisite NACK, the interrupt unexp_stop is asserted. This interrupt just means that a STOP was unexpectedly observed during a host read. It is not necessarily harmful, but software can be made aware just in case.

If the ACQ FIFO exceeds the designated number of entries, the interrupt acq_threshold is raised to inform firmware. Firmware can configure the threshold value via the register TARGET_FIFO_CONFIG.ACQ_THRESH.

Whilst the TX FIFO level is below a designated number of entries the tx_threshold interrupt is asserted. Firmware can configure the threshold value via the register TARGET_FIFO_CONFIG.TX_THRESH.

If firmware sets the bit TARGET_FIFO_CONFIG.TXRST_ON_COND, the TX FIFO will be reset whenever a RSTART or STOP condition is seen on the bus during an active Target-Mode transaction. This behaviour may be useful to software, as any remaining data in the TXFIFO after a Sr/P condition is probably no-longer applicable to the next transfer, so will likely have to be cleared out anyway. Keeping this behaviour as a toggle allows software to observe the fifo state before resetting it, which may be useful to understand how much of the previous transfer completed if that information is helpful or relevant.

If the Target module stretches the clock as a target-receiver, the interrupt acq_stretch is asserted. This can be due to a full ACQ FIFO, or it can be due to an ACK Control Mode stretch request. If ACK Control Mode is enabled, check the relevant bits in STATUS to determine the reason(s). The acq_stretch interrupt is a Status type, so it will only de-assert once the stretch conditions are cleared.

If a host ceases to send SCL pulses at any point during an ongoing transaction, the target waits for a specified time period and then asserts the interrupt host_timeout. Host sending an address and R/W bit to all target devices, writing to the selected target, or reading from the target are examples of ongoing transactions. The time period is counted from the last low-to-high SCL transition. Firmware can configure the timeout value via the register HOST_TIMEOUT_CTRL.

Implementation Details: Format Flag Parsing

To illustrate the behavior induced by various flags added to the formatting queue, the following figure shows a simplified version of the I2C_Host state machine. In this simplified view, many sequential states have been collapsed into four sub-sequences of states (shown in square brackets) or have their names abbreviated:

  • Issue start
  • Issue stop
  • Transmit Byte
  • Read Bytes

Within each of these sub-sequences, state transitions depend only on the SDA/SCL inputs or internal timers. Each sub-sequence has a terminal event–generically labeled “[completed]” which prompts the transition to another sequence or state.

However, all transitions which are dependent on formatting flags are shown explicitly in this figure.

Similarly, the figure below shows a simplified version of the I2C_Target state machine.

In this diagram, “R/W” stands for a R/W bit value. The host is reading when R/W bit is “1” and writing when R/W bit is “0”.