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Description:
pyocdbitbang 1.0.2
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.
License
For personal and professional use. You cannot resell or redistribute these repositories in their original state.
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