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Serial Peripheral Interface (SPI) is an interface bus commonly used to send data between micro controllers and small peripherals. It is a synchronous, full duplex master-slave-based interface with a single master.

It has four logic signals,

  • SCLK: Serial Clock (output from master)
  • MOSI: Master Out Slave In (data output from master)
  • MISO: Master In Slave Out (data output from slave)
  • SS: Slave Select (often active low, output from master)

MOSI on a master connects to MOSI on a slave. MISO on a master connects to MISO on a slave. Slave Select as the name suggests is used to select one of the several slaves connected to the Master.

In this tutorial, we are going to discuss about Serial Peripheral Interface (SPI) and how to read from or write to or erase the SPI flash.

Step 1: What You Will Need

Step 2: Code

Here we demonstrate how to read from or write to or erase the SPI flash in Arty7 board.

  • Write: Please click here to have a look at the code of spi_write.
  • Read: Please click here to have a look at the code of spi_read.
  • Erase: Please click here to have a look at the code of spi_erase.

Code can be compiled and run using the following ways,


Step 3.1: Manual Method

3.1.1 Compile and build

  • Move to shakti-sdk
cd shakti-sdk
  • Compile using make software PROGRAM=? TARGET=?. Type make list_applns to list all the applications available in the SDK. Target can be either parashu, pinaka or vajra. Here we use parashu.

SPI Erase:

make software PROGRAM=spierase TARGET=parashu

SPI Read:

make software PROGRAM=spiread TARGET=parashu

SPI Write:

make software PROGRAM=spiwrite TARGET=parashu

3.1.2 Execution:

Once the application is built, the executable is generated in the output folder. The executable is in ELF file format and they have the extension .shakti. Now, open three terminals, one for each of the following,

  • One terminal for UART terminal display (miniterm)
  • Another for GDB server.
  • And the last one for OpenOCD.

Terminal 1: Firstly, Connect to serial output by using miniterm or gtkterm with the baud rate of 19200.

For instance,

$ sudo miniterm.py /dev/ttyUSB1 19200

Note:

  • “/dev/ttyUSB1” – ttyUSB means “USB serial port adapter”
  • The “1” (“0” or “1” or “2”“here means the USB device number on your system. The FPGA board is connected to that USB device number.

Terminal 2: After that, Connect to the FPGA board by using the OpenOCD provided by shakti-tools and its respective configuration file.  (Read how OpenOCD and RISC-V GDB work together to establish a connection between our PC and the Microprocessor)

For instance, if we use parashu,

$ cd shakti-sdk
$ cd ./bsp/third_party/parashu
$ sudo $(which openocd) -f ftdi.cfg

For pinaka and vajra, Goto cd ./bsp/third_party/<pinaka or vajra>


Terminal 3: Now, open either 32-bit or 64-bit RISC-V GDB based on your architecture i.e riscv32-unknown-elf-gdb or riscv64-unknown-elf-gdb respectively from shakti-tools. Load up the .shakti file by following the below steps,

$ riscv64-unknown-elf-gdb
(gdb) set remotetimeout unlimited
(gdb) target remote localhost:3333
(gdb) file path/to/executable
(gdb) load
(gdb) c

4.1.3 Upload to flash

Steps to generate standalone user application:

The SHAKTI-SDK has a uploader tool that is used to load a content (such as ELF) to flash, after building the image.

  • Goto the right directory.
    cd shakti-sdk
  • The make upload command is used to build and upload the application to the flash automatically.

SPI Erase:

make upload PROGRAM=spierase TARGET=parashu

SPI Read:

make upload PROGRAM=spiread TARGET=parashu

SPI Write:

make upload PROGRAM=spiwrite TARGET=parashu

PROGRAM is the new bare metal user application that is created. Type make list_applns to list all the applications available in the SDK.

Step 4.2: Using Platform IO

We can use IDE’s like Platform IO to compile, build, run, upload and debug. Please read the article on Platform IO on SHAKTI to understand the steps.


Miniterm Output:

Erase:

Read:

WRITE:

This routine contains erase, write then read functions of SPI. First we erase the Flash. Then write to it and read from it.

--- Miniterm on /dev/ttyUSB1: 19200,8,N,1 ---
--- Quit: Ctrl+]  |  Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---                  ((                                          
                                    ,(((((
                                *((((((
                           ./(((((((
                       ./((((((((
                    *(((((((((
                .(((((((((((
             ,((((((((((((
           ((((((((((((((
          .((((((((((((((
              *(((((((((((
                   /(((((((
                  *((((((((
                     *(((((((
                       (((((((((
                        ((((((((((
                         (((((((((((((
                         ((((((((((((((
                        ((((((((((((
                      *(((((((((( 
                    (((((((((               
              *(((((((
           *((((((
       .(((((
   ./(((  
SHAKTI PROCESSORS 
Booting… vspi1.0
Booting on Parashu! hart 0 
Parashu is a SoC build on top of Artix7 100T.
The core belongs to Shakti E class, 32 bit.
Supported ISA: RV32ACIMNU.
Processor Arch ID: 0.
Device ID 1 
extracted device id 2018 
……SPI init done

Before Erase
Reading from adddress b00004 and data                297
Reading from adddress b00008 and data                9028293
Reading from adddress b0000c and data                30529073
Reading from adddress b00010 and data                41014081
Reading from adddress b00014 and data                42014181
Reading from adddress b00018 and data                43014281
Reading from adddress b0001c and data                44014381
Reading from adddress b00020 and data                5014481
Reading from adddress b00024 and data                46014581
Reading from adddress b00028 and data                47014681
Reading from adddress b0002c and data                48014781

After Erase
Reading from adddress b00004 and data                ffffffff
Reading from adddress b00008 and data                ffffffff
Reading from adddress b0000c and data                ffffffff
Reading from adddress b00010 and data                ffffffff
Reading from adddress b00014 and data                ffffffff
Reading from adddress b00018 and data                ffffffff
Reading from adddress b0001c and data                ffffffff
Reading from adddress b00020 and data                ffffffff
Reading from adddress b00024 and data                ffffffff
Reading from adddress b00028 and data                ffffffff
Reading from adddress b0002c and data                ffffffff

Flash write done on address b00004 and data deadbeef  
After Write
Reading from adddress b00004 and data                12345678
Reading from adddress b00008 and data                aaaaaaaa
Reading from adddress b0000c and data                55555555
Reading from adddress b00010 and data                aaaa5555
Reading from adddress b00014 and data                5555aaaa
Reading from adddress b00018 and data                aaa5aaa5
Reading from adddress b0001c and data                555a555a
Reading from adddress b00020 and data                aa55aa55
Reading from adddress b00024 and data                55aa55aa
Reading from adddress b00028 and data                5a5a5a5a
Reading from adddress b0002c and data                a5a5a5a5

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