Softshell

Multicore RISC-V MCU for developing software defined peripherals. This design is targetted for the first Google-sponsored MPW shuttle Nov/Dec 2020.

Configuration

CPU / Private Peripheral Configuration

• 3 x 50 MHz picorv32 cores
• 32 word private "core coupled" memory for stack
• 32-bit private GPIO peripheral
• "flexio" custom instruction (high speed configurable 2/4/8-bit shift register)

Shared Peripherals

• Round-robin arbiter
• 512 word (2KB) shared memory
• XIP QSPI flash controller
• UART
• Pinmux crossbar

Memory Map

CPU Private

``` // Local memory (for stack, etc). parameter CCM_ADDR_MASK = 32'hffff_0000; parameter CCM_BASE_ADDR = 32'h0000_0000;

// Local GPIO peripheral. parameter GPIO_ADDR_MASK = 32'hffff_0000; parameter GPIO_BASE_ADDR = 32'h2000_0000;

// Access to the shared peripheral space. parameter SHARED_ADDR_MASK = 32'hff00_0000; parameter SHARED_BASE_ADDR = 32'h3000_0000; ```

Shared

``` // Softshell base address (used for filtering addresses from Caravel). parameter SOFTSHELL_MASK = 32'hff00_0000; parameter SOFTSHELL_ADDR = 32'h3000_0000;

// Slave base addresses. parameter SHARED_RAM_MASK = 32'hfff0_0000; parameter SHARED_RAM_ADDR = 32'h3000_0000;

parameter SHARED_FLASH_MASK = 32'hfff0_0000; parameter SHARED_FLASH_ADDR = 32'h3040_0000;

parameter FLASH_CONFIG_MASK = 32'hffff_0000; parameter FLASH_CONFIG_ADDR = 32'h3080_0000; parameter PINMUX_ADDR_MASK = 32'hffff_0000; parameter PINMUX_BASE_ADDR = 32'h3081_0000; parameter UART0_ADDR_MASK = 32'hffff_0000; parameter UART0_BASE_ADDR = 32'h3082_0000; ```

Pins and Debug

``` User IOs

io[37:6] - Mapped to 32-bit pinmux (gpios, flexio, uart) io[8] - Flash CSB (muxable to Caravel passthru for programming) io[9] - Flash CLK (muxable to Caravel passthru for programming) io[10] - Flash DIO0 (muxable to Caravel passthru for programming) io[11] - Flash DIO1 (muxable to Caravel passthru for programming) io[12] - Flash DIO2 io[13] - Flash DIO3

Wishbone Access from Caravel

Access to entire shared memory space (RAM, XIP Flash, etc)

Debug from Caravel

la_data_in[0] - Wishbone reset (also resets CPUs) la_data_in[1] - CPU0 reset la_data_in[2] - CPU1 reset la_data_in[3] - CPU2 reset (if implemented) la_data_in[4] - CPU3 reset (if implemented)

la_data_out[31:0] - GPIO out la_data_out[63:32] - GPIO in

Note: CPU resets are not synchronized while the wishbone reset is. ```

Errata / History

Rev A1

Rev A1 final GDS was taped out at commit 93e9d46f5979d137ce86648ef265781f8aa0ad20 on Dec 18, 2020.

Cannot use Caravel Logic Analyzer bits [4:0]

Caravel logic analyzer connections to bits [4:0] cannot be read because they need to be used to control the Softshell resets. This is not a huge concern since these are just used to monitor the Softshell GPIO out signals and these pins tend to be used by the Caravel SoC.

Workarounds: None.

Verification

The softshell_top and user_proj_example modules are verified using the same testbench.

It performs these basic tests: - Memory read/write tests from Caravel SoC wishbone interface. - Memory tests from Softshell CPU 0. - GPIO in / out tests from Softshell CPU 0. - Flexio tests. - Flash XIP execution.

Testing execution from each CPU is a manual process requiring manually editing the linker script, flash memory load address, and which CPU is released from reset in the test bench. Not holding unused CPUs in reset will cause the X's to propagate everywhere in GL sims.

1. Build the picorv32 toolchain and install at /opt/riscv32ic/bin. We only need the riscv32ic mach.
2. Configure PDK_ROOT.

3. Run the testbench. ``` # For behavioral sims. cd verilog/rtl/softshell/dv && make clean; make

   For GL (this will take a long time).

   cd verilog/rtl/softshell/dv && make clean; make GL=1 ```

There are plans to do a full top level Caravel testbench soon.

Building

Due to the limited timeline, this design is built with two different versions of openlane / PDKs. You'll need to have both mpw-one-a and mpw-one-b available on your machine to do a full build. Note that mpw-one-a corresponds to OPENLANE_TAG=rc5.

Softshell user_proj_example and Wrapper user_project_wrapper Macros

Requires mpw-one-a openlane and PDK. To make the routing work harder, it's preferred to edit the openlane tritonRoute.param file and add drouteEndIterNum:\${ROUTING_OPT_ITERS} which we later set to try harder during wrapper routing.

1. Configure OPENLANE_ROOT and PDK_ROOT to point to mpw-one-a versions.
2. Harden the macro, this will take 8 - 12 hours on a modern 12 core machine. cd openlane && make user_proj_example OPENLANE_TAG=rc5
3. Review the final_summary_report.csv file. Expect LVS errors only on the output pins of the macro (pin mismatches) and a few antenna violations. There should be zero DRC errors, if you get some you will need to rerun to get a different seed (or manually edit the GDS). Manually review the log files to confirm that the antenna ratios are reasonable and that any LVS errors are safe (pin mismatches are generally safe, compare the GL netlist and spice extracted netlist to confirm).
4. Copy the results from the run folder into the appropriate locations. This is done primarily so it's easy to switch between run snapshots to compare. cd openlane/user_proj_example && ./copy_results.sh runs/<result tag>
5. Harden the wrapper macro. cd openlane && make user_project_wrapper OPENLANE_TAG=rc5
6. Review the final_summary_report.csv file to confirm 0 DRC/LVS/antenna violations.

Top Level Caravel

Requires mpw-one-b openlane and PDK. Probably not strictly necessary but safest to use what eFabless will use.

1. Configure OPENLANE_ROOT and PDK_ROOT to point to mpw-one-b versions.
2. Extract and build the final caravel gds. make uncompress && make ship
3. Check results, and recompress. This will take a while since we use xz compression. make compress

Caravel Top Level Wrapper Details

The top level wrapper for this project is the Caravel harness. See Caravel Readme for more information.