SPI Bus Interface

The SPI bus interface carries logical low-level SPI bus signals.

Files

Name	Description
library/spi_engine/interfaces/spi_engine_rtl.xml	Interface definition file

Signal Pins

Name	Direction (Controller)	Description
sclk	Output	SPI Clock.
sdo	Output	SPI SDO (MOSI) signal.
sdo_t	Output	sdo tri-state enable. If 1 the MOSI signal should be tristated and not be connected to sdo
sdi	Input	SPI SDI (MISO) signal. Execution module supports max 8 individual sdi lines.
cs	Output	SPI chip-select signal.
three_wire	Output	If set to 1 the bus should operate in three-wire mode. In three-wire mode sdi is connected to MOSI instead of MISO.

IO configuration

The SPI bus interface only carries a logical representation of the low-level SPI bus signals. The top-level module in the FPGA design project is responsible for translating the signal to physical SPI bus signals by instantiating and connecting it to appropriate IO primitives.

The sclk and cs signals can typically be directly connected to a output buffer. sdi should be connected to a mux, that depending on the setting of the three_wire signal connects to a input buffer connected to the miso signal or to a input buffer connected to the mosi signal. The sdo signal should be connected to a output tri-state buffer with the sdo_t signal controlling the tri-state setting.

In some configurations three-wire support may not be required and sdi can directly be connected to the input buffer for the miso signal. Similarly when mosi tri-stating is not required the sdo signal can be directly connected to the mosi signal leaving the sdo_t signal unconnected.

Example Verilog IO configuration

The following example Verilog code shows the most generic IO configuration, which represents the diagram above. Depending on system requirements some simplification might be possible.

Signals with phy prefix are assumed to be connected to the physical input/output pins and signals with the spi prefix are assumed to be connected SPI-Engine bus interface.

assign phy_sclk = spi_sclk;
assign phy_cs = spi_cs;
assign phy_mosi = spi_sdo_t ? 1'bz : spi_sdo;
assign spi_sdi = spi_three_wire ? phy_mosi : phy_miso;