diff options
Diffstat (limited to 'decoders/mfm')
| -rw-r--r-- | decoders/mfm/__init__.py | 38 | ||||
| -rw-r--r-- | decoders/mfm/pd.py | 169 |
2 files changed, 207 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 diff --git a/decoders/mfm/pd.py b/decoders/mfm/pd.py new file mode 100644 index 0000000..59ff62b --- /dev/null +++ b/decoders/mfm/pd.py @@ -0,0 +1,169 @@ +## +## 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/>. +## + +import sigrokdecode as srd + + +class SamplerateError(Exception): + pass + + +class Decoder(srd.Decoder): + """ + Python protocol: Each datum is a tuple, with the first entry being a string + denoting its type: + - HALFBIT: Contains a second entry of either 0 or 1, the value of one half + of a bitcell + - DISCONT: The data stream dropped and there is a discontinuity + - IDX: Index pulse + """ + api_version = 3 + id = 'mfm' + name = 'FM/MFM' + longname = '(Modified) Frequency Modulation' + desc = 'Floppy disk FM/MFM raw half-bits.' + license = 'gplv2+' + inputs = ['logic'] + outputs = ['mfm'] + tags = ['Encoding', 'PC', 'Retro computing'] + channels = ( + {'id': 'data', 'name': 'RD/WD', 'desc': 'Flux pulses'}, + ) + optional_channels = ( + {'id': 'idx', 'name': 'IDX', 'desc': 'Index pulses'}, + {'id': 'enablelo', 'name': 'Enable low', 'desc': 'Decode when low'}, + {'id': 'enablehi', 'name': 'Enable high', 'desc': 'Decode when high'}, + ) + annotations = ( + ('halfbit', 'Clock and data'), + ('err', 'Clock Error'), + ) + annotation_rows = ( + ('bit', 'Clock and data', (0,)), + ('errs', 'Errors', (1,)), + ) + options = ( + { + 'id': 'data_rate', + 'desc': 'Data rate (kbps)', + 'default': 250, + 'values': ( + 125, + 150, + 250, + 300, + 500, + 1000, + 5000)}, + { + 'id': 'leading_edge', + 'desc': 'Leading edge', + 'default': 'falling', + 'values': ('falling', 'rising')}) + + def __init__(self): + self.samplerate = None + + self.wait_cond = 'f' + self.data_rate = 0.0 # Hz + + def reset(self): + self.__init__() + + def metadata(self, key, value): + if key == srd.SRD_CONF_SAMPLERATE: + self.samplerate = value + + def start(self): + rising = self.options['leading_edge'] == 'rising' + self.wait_cond = 'r' if rising else 'f' + self.data_rate = float(self.options['data_rate']) * 1000 + + self.out_ann = self.register(srd.OUTPUT_ANN) + self.out_py = self.register(srd.OUTPUT_PYTHON) + + def decode(self): + if not self.samplerate: + raise SamplerateError('Cannot decode without samplerate.') + + window_size = int(self.samplerate / self.data_rate / 2) + window_size_half = int(self.samplerate / self.data_rate / 4) + + b = 0 + bit_start = self.samplenum + consecutive_zeros = 0 + skip = window_size + + while True: + last_samplenum = self.samplenum + _, _, enLo, enHi = self.wait([ + {0: self.wait_cond}, + {'skip': skip}, + {1: self.wait_cond}]) + if self.matched[2]: + self.put(self.samplenum, self.samplenum, self.out_py, ('IDX',)) + if not (self.matched[0] or self.matched[1]): + skip -= self.samplenum - last_samplenum + continue + if enLo == 1: + self.put(self.samplenum, self.samplenum, self.out_py, + ('DISCONT',)) + self.wait({2: 'l'}) + bit_start = self.samplenum + consecutive_zeros = 0 + continue + if enHi == 0: + self.put(self.samplenum, self.samplenum, self.out_py, + ('DISCONT',)) + self.wait({3: 'h'}) + bit_start = self.samplenum + consecutive_zeros = 0 + continue + + b = int(self.matched[0]) + bit_end = self.samplenum + skip = window_size + if b: + # Wait returns with zeros aligned to the end of the + # half-bitcell and ones aligned to the center. Add half a + # window for ones to get past the previous half-bitcell. + bit_end += int(.5 * window_size) + skip += window_size_half + consecutive_zeros = 0 + else: + consecutive_zeros += 1 + if consecutive_zeros > 32: + self.put(bit_start, bit_end, self.out_py, ('DISCONT',)) + # Stop decoding until there is another pulse + self.wait({0: self.wait_cond}) + bit_start = self.samplenum + consecutive_zeros = 0 + continue + + if self.samplenum < bit_start: + # Two edges in one half-bitcell + self.put(bit_start, bit_end, self.out_ann, + [1, ['Spurious pulse', 'extra']]) + elif consecutive_zeros > 4: # Allow MMFM + self.put(bit_start, bit_end, self.out_ann, + [1, ['No clock', 'clk']]) + + self.put(bit_start, bit_end, self.out_ann, [0, [str(b)]]) + self.put(bit_start, bit_end, self.out_py, ('HALFBIT', b)) + bit_start = bit_end |