2016-07-20 08:59:39 +08:00
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##
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## This file is part of the libsigrokdecode project.
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##
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## Copyright (C) 2012 Uwe Hermann <uwe@hermann-uwe.de>
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##
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## This program is free software; you can redistribute it and/or modify
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## it under the terms of the GNU General Public License as published by
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## the Free Software Foundation; either version 2 of the License, or
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## (at your option) any later version.
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##
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## This program is distributed in the hope that it will be useful,
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## but WITHOUT ANY WARRANTY; without even the implied warranty of
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## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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## GNU General Public License for more details.
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##
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## You should have received a copy of the GNU General Public License
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2019-09-09 00:07:19 -07:00
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## along with this program; if not, see <http://www.gnu.org/licenses/>.
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2016-07-20 08:59:39 +08:00
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##
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'''
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This decoder stacks on top of the 'spi' PD and decodes the SD card
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(SPI mode) low-level protocol.
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Most SD cards can be accessed via two different protocols/modes: SD mode
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or SPI mode.
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All SD cards are in SD mode upon powerup. They can be switched to SPI mode
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using a special method involving CMD0 (see spec). Once in SPI mode, the mode
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can no longer be changed without a power-cycle of the card.
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SPI mode properties (differences to SD mode):
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* The 'sdcard_spi' PD stacks on top of the 'spi' PD. This is not possible
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for the 'sdcard_sd' PD, as that protocol is not SPI related at all.
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Hence 'sdcard_spi' and 'sdcard_sd' are two separate PDs.
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* The state machines for SPI mode and SD mode are different.
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* In SPI mode, data transfers are byte-oriented (commands/data are multiples
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of 8 bits, with the CS# pin asserted respectively), unlike SD mode where
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commands/data are bit-oriented with parallel transmission of 1 or 4 bits.
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* While the SPI mode command set has some commands in common with the
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SD mode command set, they are not the same and also not a subset/superset.
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Some commands are only available in SD mode (e.g. CMD2), some only
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in SPI mode (e.g. CMD1).
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* Response types of commands also differ between SD mode and SPI mode.
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E.g. CMD9 has an R2 response in SD mode, but R1 in SPI mode.
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* The commands and functions in SD mode defined after version 2.00 of the
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spec are NOT supported in SPI mode.
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* SPI mode: The selected SD card ALWAYS responds to commands (unlike SD mode).
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* Upon data retrieval problems (read operations) the card will respond with
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an error response (and no data), as opposed to a timeout in SD mode.
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* SPI mode: For every data block sent to the card (write operations) the host
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gets a data response token from the card.
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* SDSC: A data block can be max. one card write block, min. 1 byte.
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* SDHC/SDXC: Block length is fixed to 512. The block length set by CMD16
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is only used for CDM42 (not for memory data transfers). Thus, partial
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read/write operations are disabled in SPI mode.
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* SPI mode: Write protected commands (CMD28, CMD29, CMD30) are not supported.
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* The SD mode state machine is NOT used. All commands that are supported
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in SPI mode are always available.
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* Per default the card is in CRC OFF mode. Exception: CMD0 (which is used to
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switch to SPI mode) needs a valid CRC.
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* The APP_CMD status bit is not available in SPI mode.
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* TODO: Switch function command differences.
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* In SPI mode cards cannot guarantee their speed class (the host should
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assume class 0, no matter what the card indicates).
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* The RCA register is not accessible in SPI mode.
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'''
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from .pd import Decoder
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