PULPissimo

PULPissimo is the microcontroller architecture of the more recent PULP chips, part of the ongoing "PULP platform" collaboration between ETH Zurich and the University of Bologna - started in 2013.

PULPissimo, like PULPino, is a single-core platform. However, it represents a significant step ahead in terms of completeness and complexity with respect to PULPino - in fact, the PULPissimo system is used as the main System-on-Chip controller for all recent multi-core PULP chips, taking care of autonomous I/O, advanced data pre-processing, external interrupts, etc. The PULPissimo architecture includes:

• Either the RI5CY core or the Ibex one as main core
• Autonomous Input/Output subsystem (uDMA)
• New memory subsystem
• Support for Hardware Processing Engines (HWPEs)
• New simple interrupt controller
• New peripherals
• New SDK

RISCY is an in-order, single-issue core with 4 pipeline stages and it has an IPC close to 1, full support for the base integer instruction set (RV32I), compressed instructions (RV32C) and multiplication instruction set extension (RV32M). It can be configured to have single-precision floating-point instruction set extension (RV32F). It implements several ISA extensions such as: hardware loops, post-incrementing load and store instructions, bit-manipulation instructions, MAC operations, support fixed-point operations, packed-SIMD instructions and the dot product. It has been designed to increase the energy efficiency of in ultra-low-power signal processing applications. RISCY implementes a subset of the 1.10 privileged specification. It includes an optional PMP and the possibility to have a subset of the USER MODE. RISCY implement the RISC-V Debug spec 0.13. Further information about the core can be found at http://ieeexplore.ieee.org/abstract/document/7864441/ and in the documentation of the IP.

Ibex, formely Zero-riscy, is an in-order, single-issue core with 2 pipeline stages. It has full support for the base integer instruction set (RV32I version 2.1) and compressed instructions (RV32C version 2.0). It can be configured to support the multiplication instruction set extension (RV32M version 2.0) and the reduced number of registers extension (RV32E version 1.9). Ibex implementes the Machine ISA version 1.11 and has RISC-V External Debug Support version 0.13.2. Ibex has been originally designed at ETH to target ultra-low-power and ultra-low-area constraints. Ibex is now maintained and further developed by the non-for-profit community interest company lowRISC. Further information about the core can be found at http://ieeexplore.ieee.org/document/8106976/ and in the documentation of the IP at https://ibex-core.readthedocs.io/en/latest/index.html

PULPissimo includes a new efficient I/O subsystem via a uDMA (micro-DMA) which communicates with the peripherals autonomously. The core just needs to program the uDMA and wait for it to handle the transfer. Further information about the core can be found at http://ieeexplore.ieee.org/document/8106971/ and in the documentation of the IP.

PULPissimo supports I/O on interfaces such as:

• SPI (as master)
• I2S
• Camera Interface (CPI)
• I2C
• UART
• JTAG

PULPissimo also supports integration of hardware accelerators (Hardware Processing Engines) that share memory with the RI5CY core and are programmed on the memory map. An example accelerator, performing multiply-accumulate on a vector of fixed-point values, can be found in ips/hwpe-mac-engine (after updating the IPs: see below in the Getting Started section). The ips/hwpe-stream and ips/hwpe-ctrl folders contain the IPs necessary to plug streaming accelerators into a PULPissimo or PULP system on the data and control plane. For further information on how to design and integrate such accelerators, see ips/hwpe-stream/doc and https://arxiv.org/abs/1612.05974.

Getting Started

Prerequisites

To be able to use the PULPissimo platform, you need to have installed the software development kit for PULP/PULPissimo.

First install the system dependencies indicated here: https://github.com/pulp-platform/pulp-builder/blob/master/README.md

In particular don't forget to set PULP_RISCV_GCC_TOOLCHAIN.

Then execute the following commands:

git clone https://github.com/pulp-platform/pulp-builder.git
cd pulp-builder
git checkout 0e51ae60d66f4ec326582d63a9fcd40ed2a70e15
source configs/pulpissimo.sh
./scripts/clean
./scripts/update-runtime
./scripts/build-runtime
source sdk-setup.sh
source configs/rtl.sh
cd ..

Building the RTL simulation platform

To build the RTL simulation platform, start by getting the latest version of the IPs composing the PULP system:

./update-ips

This will download all the required IPs, solve dependencies and generate the scripts by calling ./generate-scripts.

After having access to the SDK, you can build the simulation platform by doing the following:

source setup/vsim.sh
make clean build

This command builds a version of the simulation platform with no dependencies on external models for peripherals. See below (Proprietary verification IPs) for details on how to plug in some models of real SPI, I2C, I2S peripherals.

Downloading and running tests

Finally, you can download and run the tests; for that you can checkout the following repositories:

Runtime tests: https://github.com/pulp-platform/pulp-rt-examples

Now you can change directory to your favourite test e.g.: for an hello world test, run

cd pulp-rt-examples/hello
make clean all run

The open-source simulation platform relies on JTAG to emulate preloading of the PULP L2 memory. If you want to simulate a more realistic scenario (e.g. accessing an external SPI Flash), look at the sections below.

In case you want to see the Modelsim GUI, just type

make run gui=1

before starting the simulation.

If you want to save a (compressed) VCD for further examination, type

make run vsim/script=export_run.tcl

before starting the simulation. You will find the VCD in build/<SRC_FILE_NAME>/pulpissimo/export.vcd.gz where <SRC_FILE_NAME> is the name of the C source of the test.

Building and using the virtual platform

Once the RTL platform is installed, the following commands can be executed to install and use the virtual platform:

git clone https://github.com/pulp-platform/pulp-builder.git
cd pulp-builder
git checkout 7bd925324fcecae2aad9875f4da45b27d8356796
source configs/pulpissimo.sh
./scripts/build-gvsoc
source sdk-setup.sh
source configs/gvsoc.sh
cd ..

Then tests can be compiled and run as for the RTL platform. When switching from one platform to another, it may be needed to regenrate the test configuration with this command:

make conf

More information is available in the documentation here: pulp-builder/install/doc/vp/index.html

FPGA

PULPissimo has been implemented on FPGA for the various Xilinx FPGA boards.

Supported Boards

At the moment the following boards are supported:

• Digilent Genesys2
• Xilinx ZCU104
• Xilinx ZCU102
• Digilent Nexys Video
• ZedBoard

In the release section you find precompiled bitstreams for all of the above mentionied boards. If you want to use the latest development version PULPissimo follow the section below to generate the bitstreams yourself.

Bitstream Generation

In order to generate the PULPissimo bitstream for a supported target FPGA board first generate the necessary synthesis include scripts by starting the update-ips script in the pulpissimo root directory:

./update-ips

This will parse the ips_list.yml using the PULP IPApproX IP management tool to generate tcl scripts for all the IPs used in the PULPissimo project. These files are later on sourced by Vivado to generate the bitstream for PULPissimo.

Now switch to the fpga subdirectory and start the apropriate make target to generate the bitstream:

cd fpga
make <board_target>

In order to show a list of all available board targets call:

make help

This process might take a while. If everything goes well your fpga directory should now contain two files:

• pulpissimo_<board_target>.bit the bitstream file for JTAG configuration of the FPGA.
• pulpissimo_<board_target>.bin the binary configuration file to flash to a non-volatile configuration memory.

If your invocation command to start Vivado isn't vivado you can use the Make variable VIVADO to specify the right command (e.g. make genesys2 VIVADO='vivado-2018.3 vivado' for ETH CentOS machines.) Boot from ROM is not available yet. The ROM will always return the jal x0,0 to trap the core until the debug module takes over control and loads the programm into L2 memory. Once the bitstream pulpissimo_genesys2.bit is generated in the fpga folder, you can open Vivado vivado (we tried the 2018.3 version) and load the bitstream into the fpga or use the Configuration File (pulpissimo_genesys2.bin) to flash it to the on-board Configuration Memory.

Bitstream Flashing

Start Vivado then:

Open Hardware Manager
Open Target
Program device

Now your FPGA is ready to emulate PULPissimo!

Board Specific Information

Have a look at the board specific README.md files in fpga/pulpissimo-<board_target>/README.md for a description of peripheral mappings and default clock frequencies.

Compiling Applications for the FPGA Target

To run or debug applications for the FPGA you need to use a recent version of the PULP-SDK (commit id 3256fe7 or newer.'). Configure the SDK for the FPGA platform by running the following commands within the SDK's root directory:

source configs/pulpissimo.sh
source configs/fpgas/pulpissimo/<board_target>.sh

If you updated the SDK don't forget to recompile the SDK and the dependencies.

In order for the SDK to be able to configure clock dividers (e.g. the ones for the UART module) to the right values it needs to know which frequencies PULPissimo is running at. You can find the default frequencies in the above mentioned board specific README files.

In our application we need to override two weakly defined variables in our source code to configure the SDK to use these frequencies:

#include <stdio.h>
#include <rt/rt_api.h>

int __rt_fpga_fc_frequency = <Core Frequency> // e.g. 20000000 for 20MHz;
int __rt_fpga_periph_frequency = <SoC Frequency> // e.g. 10000000 for 10MHz;

int main()
{
...
}

By default, the baudrate of the UART is set to 115200.

Add the following global variable declaration to your application in case you want to change it:

unsigned int __rt_iodev_uart_baudrate = your baudrate;

Compile your application with

make clean all

This command builds the ELF binary with UART as the default io peripheral. The binary will be stored at build/pulpissimo/[app_name]/[app_name].

Core selection

By default, PULPissimo is configured to use the RI5CY core with floating-point support being enabled. To switch to Ibex (and disable floating-point support), the following steps need to be performed.

1. Switch hardware configuration

   Open the file fpga/pulpissimo-<board_target>/rtl/xilinx_pulpissimo.v and change the CORE_TYPE parameter to the preferred value. Change the value of the USE_FPU parameter from 1 to 0. Save the file and regenerate the FPGA bitstream.

2. Switch SDK configuration

   Instead of sourcing configs/pulpissimo.sh when configuring the SDK, source configs/pulpissimo_ibex.sh.

GDB and OpenOCD

In order to execute our application on the FPGA we need to load the binary into PULPissimo's L2 memory. To do so we can use OpenOCD in conjunction with GDB to communicate with the internal RISC-V debug module.

PULPissimo uses JTAG as a communication channel between OpenOCD and the Core. Have a look at the board specific README file on how to connect your PC with PULPissimo's JTAG port.

Due to a long outstanding issue in the RISC-V OpenOCD project (issue #359) the riscv/riscv-openocd does not work with PULPissimo. However there is a small workaround that we incorporated in a patched version of openocd. If you have access to the artifactory server, the patched openocd binary is installed by default with the make deps command in the SDK. If you don't have access to the precompiled binaries you can automatically download and compile the patched OPENOCD from source. You will need to install the following dependencies on your machine before you can compile OpenOCD:

• autoconf >= 2.64
• automake >= 1.14
• texinfo
• make
• libtool
• pkg-config >= 0.23 (or compatible)
• libusb-1.0
• libftdi
• libusb-0.1 or libusb-compat-0.1 for some older drivers

After installing those dependecies with you OS' package manager you can download, apply the patch and compile OpenOCD with:

source sourceme.sh && ./pulp-tools/bin/plpbuild checkout build --p openocd --stdout

The SDK will automatically set the environment variable OPENOCD to the installation path of this patched version.

Launch openocd with one of the provided or your own configuration file for the target board as an argument.

E.g.:

$OPENOCD/bin/openocd -f pulpissimo/fpga/pulpissimo-genesys2/openocd-genesys2.cfg

In a seperate terminal launch gdb from your pulp_riscv_gcc installation passing the ELF file as an argument with:

$PULP_RISCV_GCC_TOOLCHAIN_CI/bin/riscv32-unknown-elf-gdb PATH_TO_YOUR_ELF_FILE

In gdb, run:

(gdb) target remote localhost:3333

to connect to the OpenOCD server.

In a third terminal launch a serial port client (e.g. screen or minicom) on Linux to riderect the UART output from PULPissimo with e.g.:

screen /dev/ttyUSB0 115200

the ttyUSB0 target may change.

Now you are ready to debug!

In gdb, load the program into L2:

(gdb) load

and run the programm:

(gdb) continue

Of course you can also benefit from the debug capabilities that GDB provides.

E.g. see the disasembled binary:

(gdb) disas

List the current C function, set a break point at line 25, continue and have fun!

(gdb) list
21
22  int main()
23  {
24    while (1) {
25      printf("Hello World!\n\r");
26     for (volatile int i=0; i<1000000; i++);
27    }
28    return 0;
29  }

(gdb) b 25
Breakpoint 1 at 0x1c0083d2: file test.c, line 25.
(gdb) c
Continuing.

Breakpoint 1, main () at test.c:25
25      printf("Hello World!\n\r");


(gdb) disas
Dump of assembler code for function main:
   0x1c0083d4 <+22>:    li  a1,1
   0x1c0083d6 <+24>:    blt s0,a5,0x1c0083e8 <main+42>
=> 0x1c0083da <+28>:    lw  a5,12(sp)
   0x1c0083dc <+30>:    slli    a1,a1,0x1
   0x1c0083de <+32>:    addi    a5,a5,1
   0x1c0083e0 <+34>:    sw  a5,12(sp)

(gdb) monitor reg a5
a5 (/32): 0x000075B7

Not all gdb commands work as expected on the riscv-dbg target. To get a list of available gdb commands execute:

monitor help

Most notably the command info registers does not work. Use monitor reg instead which has the same effect.

Proprietary verification IPs

The full simulation platform can take advantage of a few models of commercial SPI, I2C, I2S peripherals to attach to the open-source PULP simulation platform. In rtl/vip/spi_flash, rtl/vip/i2c_eeprom, rtl/vip/i2s you find the instructions to install SPI, I2C and I2S models.

When the SPI flash model is installed, it will be possible to switch to a more realistic boot simulation, where the internal ROM of PULP is used to perform an initial boot and to start to autonomously fetch the program from the SPI flash. To do this, the LOAD_L2 parameter of the testbench has to be switched from JTAG to STANDALONE.

PULP platform structure

After being fully setup as explained in the Getting Started section, this root repository is structured as follows:

• rtl/tb contains the main platform testbench and the related files.
• rtl/vip contains the verification IPs used to emulate external peripherals, e.g. SPI flash and camera.
• rtl could also contain other material (e.g. global includes, top-level files)
• ips contains all IPs downloaded by update-ips script. Most of the actual logic of the platform is located in these IPs.
• sim contains the ModelSim/QuestaSim simulation platform.
• pulp-sdk contains the PULP software development kit; pulp-sdk/tests contains all tests released with the SDK.
• ipstools contains the utils to download and manage the IPs and their dependencies.
• ips_list.yml contains the list of IPs required directly by the platform. Notice that each of them could in turn depend on other IPs, so you will typically find many more IPs in the ips directory than are listed in this file.
• rtl_list.yml contains the list of places where local RTL sources are found (e.g. rtl/tb, rtl/vip).

Requirements

The RTL platform has the following requirements:

• Relatively recent Linux-based operating system; we tested Ubuntu 16.04 and CentOS 7.
• Mentor ModelSim in reasonably recent version (we tested it with version 10.6b -- the free version provided by Altera is only partially working, see issue #12).
• Python 3.4, with the pyyaml module installed (you can get that with pip3 install pyyaml).
• The SDK has its own dependencies, listed in https://github.com/pulp-platform/pulp-sdk/blob/master/README.md

Repository organization

The PULP and PULPissimo platforms are highly hierarchical and the Git repositories for the various IPs follow the hierarchy structure to keep maximum flexibility. Most of the complexity of the IP updating system are hidden behind the update-ips and generate-scripts Python scripts; however, a few details are important to know:

• Do not assume that the master branch of an arbitrary IP is stable; many internal IPs could include unstable changes at a certain point of their history. Conversely, in top-level platforms (pulpissimo, pulp) we always use stable versions of the IPs. Therefore, you should be able to use the master branch of pulpissimo safely.
• By default, the IPs will be collected from GitHub using HTTPS. This makes it possible for everyone to clone them without first uploading an SSH key to GitHub. However, for development it is often easier to use SSH instead, particularly if you want to push changes back. To enable this, just replace https://github.com with git@github.com in the ipstools_cfg.py configuration file in the root of this repository.

The tools used to collect IPs and create scripts for simulation have many features that are not necessarily intended for the end user, but can be useful for developers; if you want more information, e.g. to integrate your own repository into the flow, you can find documentation at https://github.com/pulp-platform/IPApproX/blob/master/README.md

External contributions

The supported way to provide external contributions is by forking one of our repositories, applying your patch and submitting a pull request where you describe your changes in detail, along with motivations. The pull request will be evaluated and checked with our regression test suite for possible integration. If you want to replace our version of an IP with your GitHub fork, just add group: YOUR_GITHUB_NAMESPACE to its entry in ips_list.yml or ips/pulp_soc/ips_list.yml. While we are quite relaxed in terms of coding style, please try to follow these recommendations: https://github.com/pulp-platform/ariane/blob/master/CONTRIBUTING.md

Known issues

The current version of the PULPissimo platform does not include yet an FPGA port or example scripts for ASIC synthesis; both things may be deployed in the future. The ipstools includes only partial support for simulation flows different from ModelSim/QuestaSim.

Support & Questions

For support on any issue related to this platform or any of the IPs, please add an issue to our tracker on https://github.com/pulp-platform/pulpissimo/issues