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diff --git a/decoders/pca9571/pd.py b/decoders/pca9571/pd.py
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+++ b/decoders/pca9571/pd.py
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+##
+## This file is part of the libsigrokdecode project.
+##
+## Copyright (C) 2019 Mickael Bosch <mickael.bosch@linux.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
+
+NUM_OUTPUT_CHANNELS = 8
+
+# TODO: Other I²C functions: general call / reset address, device ID address.
+
+class Decoder(srd.Decoder):
+    api_version = 3
+    id = 'pca9571'
+    name = 'NXP PCA9571'
+    longname = 'NXP Semiconductors PCA9571'
+    desc = 'NXP PCA9571 8-bit I²C output expander.'
+    license = 'gplv2+'
+    inputs = ['i2c']
+    outputs = []
+    tags = ['Embedded/industrial', 'IC']
+    annotations = (
+        ('register', 'Register type'),
+        ('value', 'Register value'),
+        ('warning', 'Warning messages'),
+    )
+    annotation_rows = (
+        ('regs', 'Registers', (0, 1)),
+        ('warnings', 'Warnings', (2,)),
+    )
+
+    def __init__(self):
+        self.reset()
+
+    def reset(self):
+        self.state = 'IDLE'
+        self.last_write = 0xFF # Chip port default state is high.
+
+    def start(self):
+        self.out_ann = self.register(srd.OUTPUT_ANN)
+
+    def putx(self, data):
+        self.put(self.ss, self.es, self.out_ann, data)
+
+    def handle_io(self, b):
+        if self.state == 'READ DATA':
+            operation = ['Outputs read', 'R']
+            if b != self.last_write:
+                self.putx([2, ['Warning: read value and last write value '
+                               '(%02X) are different' % self.last_write]])
+        else:
+            operation = ['Outputs set', 'W']
+            self.last_write = b
+        self.putx([1, [operation[0] + ': %02X' % b,
+                       operation[1] + ': %02X' % b]])
+
+    def check_correct_chip(self, addr):
+        if addr != 0x25:
+            self.putx([2, ['Warning: I²C slave 0x%02X not a PCA9571 '
+                           'compatible chip.' % addr]])
+            return False
+        return True
+
+    def decode(self, ss, es, data):
+        cmd, databyte = data
+        self.ss, self.es = ss, es
+
+        # State machine.
+        if cmd in ('ACK', 'BITS'): # Discard 'ACK' and 'BITS'.
+            pass
+        elif cmd in ('START', 'START REPEAT'): # Start a communication.
+            self.state = 'GET SLAVE ADDR'
+        elif cmd in ('NACK', 'STOP'): # Reset the state machine.
+            self.state = 'IDLE'
+        elif cmd in ('ADDRESS READ', 'ADDRESS WRITE'):
+            if ((self.state == 'GET SLAVE ADDR') and
+                    self.check_correct_chip(databyte)):
+                if cmd == 'ADDRESS READ':
+                    self.state = 'READ DATA'
+                else:
+                    self.state = 'WRITE DATA'
+            else:
+                self.state = 'IDLE'
+        elif cmd in ('DATA READ', 'DATA WRITE'):
+            if self.state in ('READ DATA', 'WRITE DATA'):
+                self.handle_io(databyte)
+            else:
+                self.state = 'IDLE'