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 zero-riscy 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 1.9 privileged specification. Further information about the core can be found at http://ieeexplore.ieee.org/abstract/document/7864441/ and in the documentation of the IP.

zero-riscy is an in-order, single-issue core with 2 pipeline stages and it has full support for the base integer instruction set (RV32I) and compressed instructions (RV32C). It can be configured to have multiplication instruction set extension (RV32M) and the reduced number of registers extension (RV32E). It has been designed to target ultra-low-power and ultra-low-area constraints. zero-riscy implementes a subset of the 1.9 privileged specification. Further information about the core can be found at http://ieeexplore.ieee.org/document/8106976/ and in the documentation of the IP.

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

Then execute the following commands:

git clone https://github.com/pulp-platform/pulp-builder.git
cd pulp-builder
source configs/pulpissimo_v2.sh
./scripts/build-runtime
source sdk-setup.sh
source configs/rtl.sh

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
cd sim/
make clean lib build opt

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 conf gui=1

before starting the simulation.

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

make conf 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.

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