Initial attempt at MAX7219/MAX7221 decoder

diff --git a/decoders/max7219/__init__.py b/decoders/max7219/__init__.py
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+++ b/decoders/max7219/__init__.py
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+##
+## This file is part of the libsigrokdecode project.
+##
+## Copyright (C) 2015 Paul Evans <leonerd@leonerd.org.uk>
+##
+## 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, write to the Free Software
+## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+##
+
+'''
+This decoder stacks on top of the 'spi' PD and decodes the Maxim MAX7219 and
+MAX7221 LED matrix driver protocol.
+'''
+
+from .pd import Decoder

diff --git a/decoders/max7219/pd.py b/decoders/max7219/pd.py
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+++ b/decoders/max7219/pd.py
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+##
+## This file is part of the libsigrokdecode project.
+##
+## Copyright (C) 2015 Paul Evans <leonerd@leonerd.org.uk>
+##
+## 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, write to the Free Software
+## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+##
+
+import re
+import sigrokdecode as srd
+
+def _decode_intensity(val):
+    intensity = val & 0x0f
+    if intensity == 0:
+        return 'min'
+    elif intensity == 15:
+        return 'max'
+    else:
+        return intensity
+
+registers = {
+    0x00: ['No-op', lambda _: ''],
+    0x09: ['Decode', lambda v: '0b{:08b}'.format(v)],
+    0x0A: ['Intensity', _decode_intensity],
+    0x0B: ['Scan limit', lambda v: 1 + v],
+    0x0C: ['Shutdown', lambda v: 'off' if v else 'on'],
+    0x0F: ['Display test', lambda v: 'on' if v else 'off']
+}
+
+ann_reg, ann_digit, ann_warning = range(3)
+
+class Decoder(srd.Decoder):
+    api_version = 2
+    id = 'max7219'
+    name = 'MAX7219'
+    longname = 'Maxim MAX7219/MAX7221'
+    desc = '8-digit LED display driver.'
+    license = 'gplv2+'
+    inputs = ['spi']
+    outputs = ['max7219']
+    annotations = (
+        ('register', 'Registers written to the device'),
+        ('digit', 'Digits displayed on the device'),
+        ('warnings', 'Human-readable warnings'),
+    )
+    annotation_rows = (
+        ('commands', 'Commands', (ann_reg, ann_digit)),
+        ('warnings', 'Warnings', (ann_warning,)),
+    )
+
+    def start(self):
+        self.out_ann = self.register(srd.OUTPUT_ANN)
+        self.pos = 0
+        self.cs_start = 0
+
+    def putreg(self, ss, es, reg, value):
+        self.put(ss, es, self.out_ann, [ann_reg, ['%s: %s' % (reg, value)]])
+
+    def putdigit(self, ss, es, digit, value):
+        self.put(ss, es, self.out_ann, [ann_digit, ['Digit %d: %02X' % (digit, value)]])
+
+    def putwarn(self, ss, es, message):
+        self.put(ss, es, self.out_ann, [ann_warning, [message]])
+
+    def decode(self, ss, es, data):
+        ptype, mosi, _ = data
+
+        if ptype == 'DATA':
+            if not self.cs_asserted:
+                return
+
+            if self.pos == 0:
+                self.addr = mosi
+                self.addr_start = ss
+            elif self.pos == 1:
+                if self.addr >= 1 and self.addr <= 8:
+                    self.putdigit(self.addr_start, es, self.addr, mosi)
+                elif self.addr in registers:
+                    name, decoder = registers[self.addr]
+                    self.putreg(self.addr_start, es, name, decoder(mosi))
+                else:
+                    self.putwarn(self.addr_start, es,
+                        'Unknown register %02X' % (self.addr))
+
+            self.pos += 1
+        elif ptype == 'CS-CHANGE':
+            self.cs_asserted = mosi
+            if self.cs_asserted:
+                self.pos = 0
+                self.cs_start = ss
+            else:
+                if self.pos == 1:
+                    # Don't warn if pos=0 so that CS# glitches don't appear
+                    # as spurious warnings.
+                    self.putwarn(self.cs_start, es, 'Short write')
+                elif self.pos > 2:
+                    self.putwarn(self.cs_start, es, 'Overlong write')