JESD204B IP Core Design Example User Guide

ID 683094
Date 11/06/2017

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Document Table of Contents Serial Port Interface (SPI)

An external converter device with a SPI allows you to configure the converter for specific functions or operations through a structured register space provided inside the converter device. The SPI gives flexibility and customization, depending on the application. Addresses are accessed via the serial port and can be written to or read from the port. The memory is organized into bytes that can be further divided into fields.

The SPI communicates using two data lines, a control line, and a synchronization clock. A single SPI master can work with multiple slaves. The SPI core logic is synchronous to the clock input provided by the Avalon-MM interface. When configured as a master, the core divides the Avalon-MM clock to generate the SCLK output.

Figure 17.  Serial Port Interface (24-bit) Timing DiagramFigure shows the timing diagram of a 24-bit SPI transaction required by a typical external converter device.

The first 16 bits are instruction data. The first bit in the stream is the read or write indicator bit. This bit goes high to indicate a read request. W1 and W0 represent the number of data bytes to transfer for either a read or write process. For implementation simplicity, W1 and W0 are always set at 0 in this design example. The subsequent 13 bits represent the starting address of the data sent. The last 8 bits are register data.

For a 32-bit SPI transaction, each SPI programming cycle needs to be preceded with a preselection byte. The preselection byte is typically used to forward the SPI command to the right destination. figure shows the timing diagram of a 32-bit SPI transaction.

Figure 18.  Serial Port Interface (32-bit) Timing Diagram

In this design example, the SPI core is configured as a 4-wire master protocol to control three independent SPI slaves—ADC, DAC, and clock devices. The width of the receive and transmit registers are configured at 32 bits. Data is sent in MSB-first mode in compliance with the converter device default power up mode. The SPI clock (sclk) rate is configured at a frequency of the SPI input clock rate divided by 5. If the SPI input clock rate is 100 MHz (in the mgmt_clock domain), the sclk rate is 20 MHz. If the external converter device's SPI interface is a 3-wire protocol without both MOSI (master output, slave input) and MISO (master input, slave output) lines but with a single DATAIO pin, you can use the ALTIOBUF Megafunction IP core (configured with bidirectional buffer) with the SPI master to convert the MOSI and MISO lines to a single DATAIO pin. The DATAIO pin can be dynamically reconfigured as MOSI by asserting the output enable (oe) signal or as MISO by deasserting the oe signal. For implementation simplicity, you can directly connect the master MOSI pin to the slave DATAIO pin if read transactions are not required.