Add mfm and floppy decodersfloppy

The "MFM" decoder can handle FM, MFM, MMFM, GCR, and could be extended
to support RLL. I have only tested with FM and MFM. These are all
related encodings but "MFM" is the most distinctive name so it is
being used to describe the family. The family of encodings was used in
magnetic tape storage, but the focus here is floppies with a side of
hard drives.

The MFM decoder is pretty simple, and doesn't attempt to separate
clock/data or align bytes. The method of doing so varies per sector
format so that responsibility is left to the consumer, which makes
configuration for the user easier. The decoder also doesn't try to act
as a PLL. Someone else can enhance it, but it currently seems to work
fine even on some early-1980s floppy disks/drives.

The Floppy decoder can also be used with ST506-style hard drives...
which are not floppies. But the encoding was very similar when MFM was
in use. I have not tested an ST506 using IBM-compatible MFM formatting,
but I have tested a Micropolis-encoded HDD which is a slight variation
of the MFM FDD format.

1 files changed, 38 insertions, 0 deletions

diff --git a/decoders/mfm/__init__.py b/decoders/mfm/__init__.py
new file mode 100644
index 0000000..24dfe3b
--- /dev/null
+++ b/decoders/mfm/__init__.py
@@ -0,0 +1,38 @@
+##
+## This file is part of the libsigrokdecode project.
+##
+## Copyright (C) 2023 Eric Anderson <ejona86@gmail.com>
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 2 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; if not, see <http://www.gnu.org/licenses/>.
+##
+
+'''
+FM and MFM is a combined clock/data encoding used on floppy disks. The bit
+stream requires a stacked decoder to detect a format-specific sync pattern to
+determine the byte boundaries and to separate the clock and data.
+
+The necessary sampling frequency is often 8x the data rate, so sampling at
+least at 16x the data rate is recommended.
+
+There are various interfaces with floppy disk drives, but most are very similar
+in principal to the Shugart interface. They use open-drain signals with events
+signaled with falling edges. Decoding the data is mostly done with the Read
+Data (RDATA, RD) and Write Data (WDATA, WD) lines, with optional help from
+Index (IDX), Write Gate (WG), and other lines. The "Enable low" and "Enable
+high" channels are most directly used by setting one of the channels to WG to
+enable decoding only when writing or reading. But they can also be used with
+other signals like Disk Select, Step, Track 0, and Side Select.
+'''
+
+from .pd import Decoder