SOC Design For Testing Digital IP chips (RISC V and SRAM)

This project explains the importance of components used in the PCB design based on the RISC V and SRAM are on the single chip for testing the entire PCB board and also explains how data flows within the design right from writing the code in C in a Software IDE/SDK to transmitting the code to the Flash Memory.

Table of Contents

• Brief Introduction to Testing flow
• Booting Flow
• Introduction to FT2232H
• Data transfer from USB port to FT2232H
• Data transfer from FT2232H to SPI flash memory
• Future work
• Acknowledgements

Brief Introduction to Testing flow

An IDE ( Integrated Development Environment) is used for programming the PCB design. For an IDE to recognize the hardware board, a Board Support Package(BSP) which includes hardware description and linker files for memory mapping are used and after programming, the hex files are generated. The resultant bitstream is passed to the following PCB design.

Booting Flow

• The boot process begins at Power On Reset (POR) where the hardware reset logic forces the RISC-V to begin execution starting from the on-chip SPI Flash.
• When connecting the board to the Laptop or Desktop the medium of connection for exchanging data is usually through RS-232, but the output swing of RS-232 is between -13V to +13V which is not in case of microprocessors. In microprocessors the output swing is can range from 5.5V to 3.3V/1.8V.
• For now, let's consider the programming of the chip can be done from Freedom Studio IDE (Integrated Development Environment)
• We use a USB type A-to-B cable that can be used to connect a host system to the board for data transfer from the Local device to the board, once connected, the LEDs will glow to indicate the main 5V power supply is active along with 3.3V and 1.8V supply.
• We use a special FTDI chip (FT2232H) which converts the data from USB to SPI to transfer the data serially to the chip.
• The rate at which the data is transferred should be synchronized. That is, the sending capability of the data from the laptop to the receiving capability of the FTDI chip is termed as the BAUD rate. The BAUD rate is normally set as 9.8Mbits.
• And hence the data is received without any data loss into the FTDI chip.
• The processor can boot from an SPI flash. The device is accessed through a 4-wire SPI Interface consisting of a Serial Data Input (SI), Serial Data Output (SO), Serial Clock (SCK), and Chip Select (CSB). Instructions are sent via the SDI pin to encode instructions, addresses, or input data to the device on the rising edge of SCK.
• We can provide the simple test program with the bootloader that should print in the console and should cycle in a certain fashion to glow the LEDs on board or to display “Hello World” via UART.This test program will be overwritten in the SPI Flash when you program a new program onto the board but the bootloader code will not be modified.
• The boot starts when the CPU is released from reset (for example, on power up) and executes code in the internal SPI Flash at the reset exception address. The SPI Flash code brings the SoC out of reset and into a known state.
• The boot process ends when the code in the SPI Flash jumps to the next stage of the boot software. This next stage of boot software is referred to as the BootLoader.
• The BootLoader can be customized by mapping the internal address at the SRAM by address bus and the code is typically sent through the data bus to the SRAM and which thus stores external code from Flash memory to the internal SRAM on the chip.
• After the execution of the code, we can see the output of the program with the help of a serial terminal which is built-in in Freedom Studio IDE. The serial terminal will be displayed as a COM port. We can identify which COM port is being used by checking in the Device Manager >> Ports section of the Control Panel.
• Once the test board boots up, we can see the pattern/output we coded in the Freedom IDE software.

Introduction to FT2232H

The serial port of the computer usually meets RS-232 standard. It has signal voltage swing of around -13 to +13V. The PCB design uses TTL serial (Transistor-Transistor logic) which has signal voltage level between 0 and VDD ( 3.3V/1.8V). A common communication protocol is needed for data transfer, hence FT2232H is used for converting USB to serial converter.

For the data transfer from host to the target (board), we connect USB cable ( USB type A to the PC and Type B to the board). USB has 4 signals, 5V power supply signal, ground and 2 data signals(D+ and D-).

The data is transferred from these two data lines using NRZI encoding.

Data transfer from USB port to FT2232H

According to USB communication protocol, data is sent through packets. It consists of 3 common packets.

Data field can carry varying size of data depending upon the speed at which data is transferred.

• For low speed devices, data can be upto 8B
• For full speed devices, data can be upto 1023B
• For high speed devices, data can be upto 1024B

Basically, the data transaction can be done in 4 types:

• Control: It is generally used by host for sending commands and parameters.
• Interrupt: It is used by devices for sending small amounts of data. It is a polled message from the host to request specific data of the device
• Bulk: this is used by devices for sending large amounts of data.
• Isochronous: this is generally used by devices like audio channels etc, for streaming real time data. FTDI supports bulk data transaction. Data is sent through endpoint buffers within the device. FT2232H can have a maximum packet size of 512B.

The following example code is considered to demonstrate how the data flows from one block to other. ( Image courtesy: Shivanishah/riscv)

The example code has a bitstream of 44bytes. This is sent from host to device through data packet in data field.

The data transfer from device to the host is done when host is ready to accept data, it sends token packet. The device then sends data.

Data transfer from FT2232H to SPI flash memory

FT2232H has multiple configurations like synchronous bitbang interface, asynchronous bitbang interface and MPSSE (Multi-Protocol Synchronous Serial Engine) interface. MPSSE mode allows communication with different types of synchronous devices like SPI, I2C, JTAG etc.

SPI communication has 4 modes for data transfer depending on clock polarity (CPOL) and clock phase (CPHA). FTDI device can only support mode 0 and mode 2 whereas flash memory (S25FL128S) has only 2 clocking modes i.e mode 0 and mode 3.

The data transfer between FT2232H and S25FL128S is done in the form of units called commands. These commands consist of instructions which tells the device operation and may also have address, latency period followed by data. The input data into the device is always latched in at the rising edge of the SCK signal and the output data is always available from the falling edge of the SCK clock signal.

The memory is divided into sectors either a combination of 64KB and 4KB or a size of 256KB. For a sector size of 256KB, there can be total of 64 sectors for the memory and each sector is again divided into pages. Each page size can be a maximum of 512B.

The data is transferred in byte units and after each byte transmission the address byte is incremented automatically. For writing the data, the maximum data that can be written in one operation is upto 512B. For more data transfer, again instruction followed by address is sent.

Future work

• Data transfer from flash memory to SOC and rest of the blocks.

Acknowledgements

Kunal Ghosh, Co-founder (VSD Corp. Pvt. Ltd)