Lines Matching +full:capacitance +full:- +full:to +full:- +full:digital
5 02-Feb-2012
8 ------------
10 link used to connect microcontrollers to sensors, memory, and peripherals.
11 It's a simple "de facto" standard, not complicated enough to acquire a
17 clocking modes through which data is exchanged; mode-0 and mode-3 are most
19 doesn't cycle except when there is a data bit to shift. Not all data bits
22 SPI masters use a fourth "chip select" line to activate a given SPI slave
23 device, so those three signal wires may be connected to several chips
26 other signals, often including an interrupt to the master.
32 - SPI may be used for request/response style device protocols, as with
35 - It may also be used to stream data in either direction (half duplex),
38 - Some devices may use eight bit words. Others may use different word
39 lengths, such as streams of 12-bit or 20-bit digital samples.
41 - Words are usually sent with their most significant bit (MSB) first,
44 - Sometimes SPI is used to daisy-chain devices, like shift registers.
51 SPI is only one of the names used by such four-wire protocols, and
53 half-duplex SPI, for request/response protocols), SSP ("Synchronous
58 limiting themselves to half-duplex at the hardware level. In fact
71 ---------------------------------------
73 systems boards. SPI is used to control external chips, and it is also a
78 SPI slave chips range from digital/analog converters used for analog
79 sensors and codecs, to memory, to peripherals like USB controllers
84 dedicated SPI controller exists, GPIO pins can be used to create a
86 controller; the reasons to use SPI focus on low cost and simple operation,
88 appropriate low-pincount peripheral bus.
96 -----------------------------------------------------
97 It's easy to be confused here, and the vendor documentation you'll
100 - CPOL indicates the initial clock polarity. CPOL=0 means the
105 - CPHA indicates the clock phase used to sample data; CPHA=0 says
108 Since the signal needs to stablize before it's sampled, CPHA=0
121 active. So the master must set the clock to inactive before selecting
129 ------------------------------------------------
144 controllers may be built into System-On-Chip
151 driver to communicate with a Slave or Master device on the
154 So for example one protocol driver might talk to the MTD layer to export
155 data to filesystems stored on SPI flash like DataFlash; and others might
160 A "struct spi_device" encapsulates the controller-side interface between
164 using the driver model to connect controller and protocol drivers using
173 /sys/bus/spi/devices/spiB.C ... symlink to that physical
181 /sys/class/spi_master/spiB ... symlink (or actual device node) to
188 Writing the driver name of an SPI slave handler to this file
194 /sys/class/spi_slave/spiB ... symlink (or actual device node) to
202 the only class-specific state is the bus number ("B" in "spiB"), so
203 those /sys/class entries are only useful to quickly identify busses.
206 How does board-specific init code declare SPI devices?
207 ------------------------------------------------------
208 Linux needs several kinds of information to properly configure SPI devices.
209 That information is normally provided by board-specific code, even for
216 For System-on-Chip (SOC) based boards, these will usually be platform
217 devices, and the controller may need some platform_data in order to
222 maybe coupling it with code to initialize pin configurations, so that
223 the arch/.../mach-*/board-*.c files for several boards can all share the
225 SPI-capable controllers, and only the ones actually usable on a given
228 So for example arch/.../mach-*/board-*.c files might have code like::
232 /* if your mach-* infrastructure doesn't support kernels that can
245 And SOC-specific utility code might look something like::
259 spi2->dev.platform_data = pdata2;
265 * developer boards will often need Linux to do it.
280 on the target board, often with some board-specific data needed for the
281 driver to work correctly.
283 Normally your arch/.../mach-*/board-*.c files would provide a small table
305 Again, notice how board-specific information is provided; each chip may need
307 clock to allow (a function of board voltage in this case) or how an IRQ pin
308 is wired, plus chip-specific constraints like an important delay that's
309 changed by the capacitance at one pin.
311 (There's also "controller_data", information that may be useful to the
312 controller driver. An example would be peripheral-specific DMA tuning
315 The board_info should provide enough information to let the system work
319 not possible until the infrastructure knows how to deselect it.
327 Like with other static board-specific setup, you won't unregister those.
331 your ``arch/.../mach-.../board-*.c`` file would primarily provide information
336 Non-static Configurations
340 example is the potential need to hotplug SPI devices and/or controllers.
342 For those cases you might need to use spi_busnum_to_master() to look
343 up the spi bus master, and will likely need spi_new_device() to provide the
353 ----------------------------------------
370 The driver core will automatically attempt to bind this driver to any SPI
382 /* assuming the driver requires board-specific data: */
383 pdata = &spi->dev.platform_data;
385 return -ENODEV;
387 /* get memory for driver's per-chip state */
390 return -ENOMEM;
397 As soon as it enters probe(), the driver may issue I/O requests to
402 - An spi_message is a sequence of protocol operations, executed
420 + hinting whether the next message is likely to go to this same
425 - Follow standard kernel rules, and provide DMA-safe buffers in
427 to make extra copies unless the hardware requires it (e.g. working
431 you can use spi_message.is_dma_mapped to tell the controller driver
434 - The basic I/O primitive is spi_async(). Async requests may be
440 - There are also synchronous wrappers like spi_sync(), and wrappers
445 - The spi_write_then_read() call, and convenience wrappers around
447 cost of an extra copy may be ignored. It's designed to support
448 common RPC-style requests, such as writing an eight bit command
449 and reading a sixteen bit response -- spi_w8r16() being one its
452 Some drivers may need to modify spi_device characteristics like the
455 done to the device. However, that can also be called at any time
466 - I/O buffers use the usual Linux rules, and must be DMA-safe.
470 - The spi_message and spi_transfer metadata used to glue those
473 other allocate-once driver data structures. Zero-init these.
476 routines are available to allocate and zero-initialize an spi_message
481 -------------------------------------------------
483 a driver to bind to the device, whichever bus is involved.
485 The main task of this type of driver is to provide an "spi_master".
486 Use spi_alloc_master() to allocate the master, and spi_master_get_devdata()
487 to get the driver-private data allocated for that device.
496 return -ENODEV;
502 used to interact with the SPI core and SPI protocol drivers. It will
506 After you initialize the spi_master, then use spi_register_master() to
507 publish it to the rest of the system. At that time, device nodes for the
509 the driver model core will take care of binding them to drivers.
511 If you need to remove your SPI controller driver, spi_unregister_master()
522 and spi_board_info for devices connected to it would use that number.
524 If you don't have such hardware-assigned bus number, and for some reason
526 then be replaced by a dynamically assigned number. You'd then need to treat
527 this as a non-static configuration (see above).
533 ``master->setup(struct spi_device *spi)``
536 call spi_setup(spi) to invoke this routine. It may sleep.
545 many spi_master drivers seems to get this wrong.
549 ``master->cleanup(struct spi_device *spi)``
550 Your controller driver may use spi_device.controller_state to hold
552 be sure to provide the cleanup() method to free that state.
554 ``master->prepare_transfer_hardware(struct spi_master *master)``
555 This will be called by the queue mechanism to signal to the driver
557 driver to prepare the transfer hardware by issuing this call.
560 ``master->unprepare_transfer_hardware(struct spi_master *master)``
561 This will be called by the queue mechanism to signal to the driver
565 ``master->transfer_one_message(struct spi_master *master, struct spi_message *mesg)``
566 The subsystem calls the driver to transfer a single message while
572 ``master->transfer_one(struct spi_master *master, struct spi_device *spi, struct spi_transfer *tran…
573 The subsystem calls the driver to transfer a single transfer while
587 ``master->set_cs_timing(struct spi_device *spi, u8 setup_clk_cycles, u8 hold_clk_cycles, u8 inactiv…
588 This method allows SPI client drivers to request SPI master controller
595 ``master->transfer(struct spi_device *spi, struct spi_message *message)``
596 This must not sleep. Its responsibility is to arrange that the
599 if the controller is idle it will need to be kickstarted. This
611 providing pure process-context execution of methods. The message queue
612 can also be elevated to realtime priority on high-priority SPI traffic.
619 for low-frequency sensor access might be fine using synchronous PIO.
621 But the queue will probably be very real, using message->queue, PIO,
625 Such a transfer() method would normally just add the message to a
630 THANKS TO
631 ---------
632 Contributors to Linux-SPI discussions include (in alphabetical order,
635 - Mark Brown
636 - David Brownell
637 - Russell King
638 - Grant Likely
639 - Dmitry Pervushin
640 - Stephen Street
641 - Mark Underwood
642 - Andrew Victor
643 - Linus Walleij
644 - Vitaly Wool