pyocd-bitbang 1.0.2

PyOCD debug probe plugin for Bitbang debug probes

pyOCD RemoteBitbang DebugProbe plugin

This pyOCD plugin provides a debug probe that allows connection to a remote bitbang interface via a TCP socket. It is design to be used with targets running in simulation. The remote end must provide a TCP server that accepts a socket connection. The debug probe sends an ASCII encoding of the bitbang interface.

Installation

$ pip install pyocd-bitbang

Background

This plugin is designed to be used to connect, via SWD, to a Verilator model of a Cortex-M based MCU. This is the pyOCD equivalent to the OpenOCD JTAG remote_bitbang interface, and in fact, uses the same ASCII encoding.

Implementation Detail

DebugProbe

The class bitbang_probe:RemoteBitbangProbe implements the minimum subset required by the pyOCD pyocd.probe.debug_probe:DebugProbe class. Any SWD commands are translated to a series of read and write calls that drive the remote SWD interface via a TCP socket. The protocol used is the same as defined in the OpenOCD remote_bitbang specification. Each character sent over the socket corresponds to a request to set the bus to a particular state or to read the value currently being driven on SWDI.

Unlike debug probes connected via USB, pyOCD cannot auto-detect the probe. Instead, a unique ID must be specified when launching pyOCD that specifies the probe to use and the IP address and port to use. See below.

TCP Server and SV DPI

In addition to this plugin, the testbench must implement a TCP server to which the probe connects as well as a DPI interface that drives the SWD pins of the DUT. A good example of such an implementation can be found in the OpenTitan repository here.

Modifying the above example to drive a SWD interface is fairly simple. Instead of the 3 bit Write encoding driving {tck, tms, tdi}, they should drive {swclk, swdoen, swdo}.

The following snippet shows the character decoding in the main loop of the DPI C function:

  // parse received command byte
  if (cmd >= '0' && cmd <= '7') {
    char cmd_bit = cmd - '0';
    ctx->swclk  = (cmd_bit >> 2) & 0x1;
    ctx->swdoen = (cmd_bit >> 1) & 0x1;
    ctx->swdo   = (cmd_bit >> 0) & 0x1;
  } else if (cmd == 'R') {
    act_send_resp = true;
  } else if (cmd == 'Q') {
    act_quit = true;
  } else {
    fprintf(stderr,
      "SWD DPI Protocol violation detected: unsupported command '%c'\n", cmd);
    exit(1);
  }

Inside the remote_bitbang:BitBanger class the character encoding for writing a zero or one on the bus as well as a read is defined:

    ZERO  = b'62'
    ONE   = b'73'
    READ  = b'40R'

Usage

Assuming you have pip installed the pluggin, you should be able to connect to the target using:

$ pyocd commander -u remote_bitbang:localhost:5555

which would connect to TCP port 5555 on localhost. No further configuration is necessary.