Programmer’s Guide

The operation of the SPI_HOST IP proceeds in seven general steps.

To initialize the IP:

  1. Program the CONFIGOPTS multi-register with the appropriate timing and polarity settings for each csb line.
  2. Set the desired interrupt parameters
  3. Enable the IP

Then for each command:

  1. Load the data to be transmitted into the FIFO using the TXDATA memory window.
  2. Specify the target device by programming the CSID
  3. Specify the structure of the command by writing each segment into the COMMAND register
    • For multi-segment transactions, be sure to assert COMMAND.CSAAT for all but the last command segment
  4. For transactions which expect to receive a reply, the data can then be read back from the RXDATA window.

These latter four steps are then repeated for each command. Each step is covered in detail in the following sections.

For concreteness, this Programmer’s Guide uses examples from one of our primary target devices, the W25Q01JV flash from Winbond. The SPI_HOST IP is however suitable for interacting with any number of SPI devices, and the same mode of operation can be used for any SPI device.

Initializing the IP

Per-target Configuration

The CONFIGOPTS multi-register must be programmed to reflect the requirements of the attached target devices. As such these registers can be programmed once at initialization, or whenever a new device is connected (e.g., via changes in the external pin connections, or changes in the pinmux configuration). The proper settings for the CONFIGOPTS fields (e.g., CPOL and CPHA, clock divider, ratios, and other timing or sampling requirements) will all depend on the specific device attached as well as the board level delays.

Interrupt configuration

The next step is to configuration the interrupts for the SPI_HOST. This should also be done at initialization using the following register fields:

  • The ERROR_ENABLE register should be configured to indicate what types of error conditions (if any) should be ignored to not trigger an interrupt. At reset, these fields are all set indicating that all error classes trigger an interrupt.

  • For interrupt driven I/O the EVENT_ENABLE register must be configured to select the desired event interrupts to signal the desired conditions (e.g. “FIFO empty”, “FIFO at the watermark level”, or “ready for next command segment”). By default, this register is all zeros, meaning all event interrupts are disabled, and thus all transactions must be managed by polling the status register.

  • The event and error interrupts must finally be enabled using the INTR_ENABLE register.

Enabling the SPI_HOST

The IP must be enabled before sending the first command by asserting the CONTROL.SPIEN bit.

Issuing Transactions

As mentioned above, each command is typically specified in three phases: loading the TX data, specifying the command segments/format, and reading the RX data. In principle, the first two steps can be performed in either order. If the SPI_HOST does not find any data to transmit it will simply stall until data is inserted. Meanwhile, the RX data is only available after the command format has been specified and processed by the state machine.

For longer transactions, with data larger than the capacity of the FIFOs, the command sequence may become more complex. For instance, to send 1024 bytes of data in a single transaction, the TX data may need to be loaded several times if using a 256-byte FIFO. In this instance, the programming sequence will consist of at least four iterations of entering TX data and waiting for the TX FIFO to drain.

Loading TX data

SPI transactions expect each command to start with some command sequence from the host, and so usually data will be transmitted at least in the first command segment. The TXDATA window provides a simple interface to the TX FIFO. Data can be written to the window using 8-, 16- or 32-bit instructions.

Some attention, however, should be paid to byte-ordering and segmenting conventions.

Byte-ordering Conventions

For SPI flash applications, it is generally assumed that most of the payload data will be directly copied from embedded SRAM to the flash device.

If this data is to copied to the TXDATA window using 32-bit instructions, the SPI_HOST should be parameterized such that the ByteOrder parameter matches the byte order of the embedded CPU (i.e., for Ibex, ByteOrder should be left set to 1 to indicate a Little-Endian CPU). This will ensure that data is transmitted to the flash (and thus also stored in flash) in address-ascending order. For example, consider the transfer of four bytes, D[3:0][7:0], to SPI via the TXDATA window.

  • It is assumed for this example that all four bytes are contiguously stored in SRAM at a word-aligned address, with D[0] at the lowest byte-address.
  • When these bytes are loaded into the Ibex CPU they are arranged as the 32-bit word: W[31:0] = {D[3][7:0], D[2][7:0], D[1][7:0], D[0][7:0]}.
  • After this word are loaded into the TXDATA window, the LSB (i.e., W[7:0] = D[0][7:0]) is transmitted first, by virtue of the ByteOrder == 1 configuration.

In this way, configuring ByteOrder to match the CPU ensures that data is transmitted in memory-address order.

The value of the ByteOrder parameter can be confirmed by firmware by reading the STATUS.BYTEORDER register field.

Not all data to the SPI device will come from memory however. In many cases the transaction command codes or headers will be constructed or packed on the fly in CPU registers. The order these register bytes are transmitted on the bus will depend on the value of the ByteOrder parameter, as discussed in the Theory of Operation section, and for multi-bit values, such as flash addresses), some byte-swapping may be required to ensure that data is transmitted in the proper order expected by the target device.

For example, SPI flash devices generally expect flash addresses (or any other multi-byte values) to be transmitted MSB-first. This is illustrated in the following figure, which depicts a Fast Quad Read I/O command. Assuming that ByteOrder is set to 1 for Little-Endian devices such as Ibex, byte-swapping will be required for these addresses, otherwise the device will receive the addresses LSB first.

Byte ordering on the bus can also be managed by writing TXDATA as a sequence of discrete bytes using 8-bit transactions, since partially-filled data-words are always sent in the order they are received.

A few examples related to using SPI flash devices on a Little-Endian platform:

  • A 4-byte address can be loaded into the TX FIFO as four individual bytes using 8-bit I/O instructions.
  • The above read command (with 4-byte address) can be loaded into the FIFO by first loading the command code into TXDATA as a single byte, and the address can be loaded into TXDATA using 32-bit instructions, provided the byte order is swapped before loading.
  • Flash transactions with 3-byte addressing require some care, as there are no 24-bit I/O instructions, though there are a several options:
    • After the 8-bit command code is sent, the address can either be sent in several I/O operations (e.g., the MSB is sent as an 8-bit command, and the remaining 16-bits can be sent after swapping)
    • If bandwidth efficiency is a priority, the address, A[23:0], and command code, C[7:0], can all be packed together into a single 4-byte quantity W[31:0] = {A[7:0], A[15:8], A[23:16], C[7:0]}, which when loaded into TXDATA will ensure that the command code is sent first, followed by the address in MSB-first order.

Segmenting Considerations

Data words are not shared across segments. If at the end of each TX (or bidirectional) segment there is a partially transmitted data word then any unsent bytes will be discarded as the SPI_HOST IP closes the segment. For the next TX segment, the transmitted data will start with the following word from the TX FIFO.

Refilling the TX FIFO

For extremely long transactions, the TX FIFO may not have enough capacity to hold all the data being transmitted. In this case software can either poll the STATUS.TXQD register to determine the number of elements in the TX FIFO, or enable the SPI_HOST IP to send an interrupt when the FIFO drains to a certain level. If INTR_ENABLE.spi_event and EVENT_ENABLE.TXWM are both asserted, the IP will send an interrupt whenever the number of elements in the TX FIFO falls below CONTROL.TX_WATERMARK.

Specifying the Segments

Each write to the COMMAND register corresponds to a single command segment. The length, CSAAT flag, direction and speed settings for that segment should all be packed into a single 32-bit register and written simultaneously to COMMAND.

The COMMAND should only be written when STATUS.READY is asserted.

While each command segment is being processed, the SPI_HOST has room to queue up exactly one additional segment descriptor in the Command Clock Domain Crossing. Once a second command segment descriptor has been submitted, software must wait for the state machine to finish processing the current segment before submitting more. Software can poll the STATUS.READY field to determine when it is safe to insert another segment descriptor. Otherwise the EVENT_ENABLE.IDLE bit can be enabled (along with INTR_ENABLE.spi_event) to trigger an event interrupt whenever STATUS.READY is asserted.

Reading Back the Device Response

Once an RX segment descriptor has been submitted to the SPI_HOST, the received data will be available in the RX FIFO after the first word has been received.

The number of words in the FIFO can be polled by reading the STATUS.RXQD field. The SPI_HOST IP can also configured to generate watermark event interrupts whenever the number of words received reaches (or exceeds) CONTROL.RX_WATERMARK. To enable interrupts when ever the RX FIFO reaches the watermark, assert EVENT_ENABLE.RXWM along with INTR_ENABLE.spi_event.

Exception Handling

The SPI_HOST will assert one of the ERROR_STATUS bits in the event of a firmware programming error, and will become unresponsive until firmware acknowledges the error by clearing the corresponding error bit.

The SPI_HOST interrupt handler should clear any bits in ERROR_STATUS bit before clearing INTR_STATE.error.

In addition to clearing the ERROR_STATUS register, firmware can also trigger a complete software reset via the CONTROL.SW_RST bit, as described in the next section.

Other system-level errors may arise due to improper programming of the target device (e.g., due to violations in the device programming model, or improper configuration of the SPI_HOST timing registers). Given that the SPI protocol provides no mechanism for the target device to stall the bus, the SPI_HOST will continue to function even if the remote device becomes unresponsive. In case of an unresponsive device, the RX FIFO will still accumulate data from the bus during RX segments, though the data values will be undefined.

Software Reset Procedure

In the event of an error the SPI_HOST IP can be reset under software control using the following procedure:

  2. Poll IP status registers for confirmation of successful state machine reset:
  3. Clear CONTROL.SW_RST.

Device Interface Functions (DIFs)


Analysis of Transient Datapath Stalls

Even if the RX (or TX) FIFOs have free-space (or data) available, stall events can still happen due to momentary backlogs or bubbles in the data pipeline. For instance, the Byte Merge and Byte Select blocks occasionally need some extra cycles to clean out the internal prim_packer_fifo. These delays are likely to cause transient stalls particularly, when constructing transactions with many short (less than 4-byte) segments. Transient stalls could lead to false diagnostics when trying to optimize SPI_HOST throughput. Thus it is useful to analyze the shift register’s tolerance to bubble events, particularly for the highest bandwidth Quad SPI mode.

Transient Stalls in TX direction.

The transient analysis stall analysis is simpler for the TX direction. There is no buffering on the shift register TX data inputs because it would complicate the byte_flush operation on the Byte Select block.

In Quad mode,the shift register will demand one new byte as often as once every four clock cycles. (This rate is slowed down if for a non-trivial clock-divide ratio). Meanwhile, the Byte Select Block pauses once for every disabled byte, and once more at the end of each word. Thus if the Byte Select block is loaded with three-consecutive bytes-disables (either in the same word or across two separate words), this will create a pause of 4-clock cycles between two bytes causing a transient stall event.

Assuming that each TX Word has at least one byte enabled, the longest possible transient delay between two Byte Select outputs is 7 clock cycles (with three byte-disables in two adjacent words, respectively aligned for maximal delay and assuming no delays in the TX FIFOs themselves). Dual- and Standard-mode segments can tolerate inter-byte delays of 7 or 15 clocks respectively, and thus transient stalls should not be a problem after Dual- or Standard-mode segments.

Transient Stalls in the RX direction

Similar to the Byte Select, the Byte Merge block must pause for at least one cycle between each word. Also when at the end of a segment the Byte Merge packs less than four bytes into the last word, there is also an additional cycle of delay for each unused byte. Thus if the last word in a given segment has only one valid byte, the total delay will be four clock cycles.

Such stalls however are a much smaller concern in the RX direction due to the buffering of the Shift Register outputs. As shown in the following waveform, even in Quad-mode, this buffer means the shift register can tolerate as many as six clock cycles of temporary back-pressure before creating a stall.

Even though such long delays are tolerable, it takes some time for shift register to catch up completely and clear the backlog. For example, if after a 6-clock delay the shift-register encounters another 4-clock backlog this can also introduce a stall condition, as shown in the waveform below.

Delays of 3-clocks or less do not create any internal backlog in the system. However, the Byte Merge block can create a 4-clock delay each time it processes a single-byte segment. In practice, this is unlikely to cause a problem, as no Quad-SPI Flash transactions require even two back-to-back RX segments. However with enough (at least six) consecutive one-byte segments, the accumulated delay can eventually create a stall event on the RX path as well, as seen below.