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Ben Nuttall
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Merge pull request raspberrypi#84 from notro/spi
Add SPI documentation
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‎hardware/raspberrypi/README.md

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@@ -12,3 +12,5 @@ The hardware in the Raspberry Pi
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- Powering the Raspberry Pi
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- [USB](usb.md)
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- USB on the Raspberry Pi
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- [SPI](spi.md)
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- SPI on the Raspberry Pi

‎hardware/raspberrypi/spi.md

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# SPI
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## Page Contents
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* [Overview](#overview)
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* [Software](#software)
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* [Hardware](#hardware)
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* [Linux driver](#driver)
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* [Troubleshooting](#troubleshooting)
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## <a name="overview"></a>Overview
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The Raspberry Pi is equipped with one [SPI](http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus) bus that has 2 chip selects.
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The SPI master driver is disabled by default on Raspian. To enable it, remove the blacklisting for `spi-bcm2708` in `/etc/modprobe.d/raspi-blacklist.conf`, or use [raspi-config](../../configuration/raspi-config.md).
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Reboot or load the driver manually with:
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```
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$ sudo modprobe spi-bcm2708
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```
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The SPI bus is available on the P1 Header:
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```
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MOSI P1-19
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MISO P1-21
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SCLK P1-23 P1-24 CE0
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GND P1-25 P1-26 CE1
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```
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## <a name="software"></a>Software
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### WiringPi
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WiringPi includes a library which can make it easier to use the Raspberry Pi’s on-board SPI interface. Accesses the hardware registers directly.
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http://wiringpi.com/
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### bcm2835 library
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This is a C library for Raspberry Pi (RPi). It provides access to GPIO and other IO functions on the Broadcom BCM 2835 chip. Accesses the hardware registers directly.
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http://www.airspayce.com/mikem/bcm2835/
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### Use spidev from C
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There's a loopback test program in the Linux documentation that can be used as a starting point. See the [Troubleshooting](#troubleshooting) section. Uses the Linux `spidev` driver to access the bus.
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### Shell
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```
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# Write binary 1, 2 and 3
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echo -ne "\x01\x02\x03" > /dev/spidev0.0
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```
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## <a name="hardware"></a>Hardware
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The BCM2835 on the Raspberry Pi has 3 SPI Controllers. Only the SPI0 controller is available on the header.
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Chapter 10 in the [BCM2835 ARM Peripherals](http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf) datasheet describes this controller.
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### Master modes
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Signal name abbreviations
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```
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SCLK - Serial CLocK
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CE - Chip Enable (often called Chip Select)
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MOSI - Master Out Slave In
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MISO - Master In Slave Out
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SDA - Serial DAta
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MOMI - Master Out Master In
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MIMO - Master In Master Out
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```
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#### Standard mode
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In Standard SPI master mode the peripheral implements the standard 3 wire serial protocol (SCLK, MOSI and MISO).
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#### Bidirectional mode
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In bidirectional SPI master mode the same SPI standard is implemented except that a single wire is used for data (MIMO) instead of two as in standard mode (MISO and MOSI).
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#### LoSSI mode (Low Speed Serial Interface)
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The LoSSI standard allows issuing of commands to peripherals (LCD) and to transfer data to and from them. LoSSI commands and parameters are 8 bits long, but an extra bit is used to indicate whether the byte is a command or parameter/data. This extra bit is set high for a data and low for a command. The resulting 9-bit value is serialized to the output. LoSSI is commonly used with [MIPI DBI](http://mipi.org/specifications/display-interface) type C compatible LCD controllers.
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**Note:**
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Some commands trigger an automatic read by the SPI controller, so this mode can't be used as a multipurpose 9-bit SPI.
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### Transfer modes
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* Polled
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* Interrupt
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* DMA
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### Speed
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The CDIV (Clock Divider) field of the CLK register sets the SPI clock speed:
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```
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SCLK = Core Clock / CDIV
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If CDIV is set to 0, the divisor is 65536. The divisor must be a power of 2. Odd numbers rounded down. The maximum SPI clock rate is of the APB clock.
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```
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[Errata](http://elinux.org/BCM2835_datasheet_errata): "must be a power of 2" probably should be "must be a multiple of 2"
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See the [Linux driver](#driver) section for more info.
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### Chip Select
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The controller supports 3 Chip selects, but only 2 is available on the header.
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Setup and Hold times related to the automatic assertion and de-assertion of the CS lines when operating in **DMA** mode are as follows:
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* The CS line will be asserted at least 3 core clock cycles before the msb of the first byte of the transfer.
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* The CS line will be de-asserted no earlier than 1 core clock cycle after the trailing edge of the final clock pulse.
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## <a name="driver"></a>Linux driver
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The default Linux driver is [spi-bcm2708](https://github.com/raspberrypi/linux/blob/rpi-3.12.y/drivers/spi/spi-bcm2708.c).
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The following information was valid 2014-07-05.
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### Speed
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The driver supports the following speeds
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```
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cdiv speed
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2 125.0 MHz
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4 62.5 MHz
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8 31.2 MHz
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16 15.6 MHz
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32 7.8 MHz
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64 3.9 MHz
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128 1953 kHz
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256 976 kHz
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512 488 kHz
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1024 244 kHz
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2048 122 kHz
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4096 61 kHz
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8192 30.5 kHz
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16384 15.2 kHz
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32768 7629 Hz
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```
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When asking for say 24 MHz, the actual speed will be 15.6 MHz.
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Forum post: [SPI has more speeds](http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=43442&p=347073)
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### Supported Mode bits
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```
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SPI_CPOL - Clock polarity
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SPI_CPHA - Clock phase
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SPI_CS_HIGH - Chip Select active high
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SPI_NO_CS - 1 device per bus, no Chip Select
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```
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Bidirectional mode is currently not supported.
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### Supported bits per word
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* 8 - Normal
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* 9 - This is supported using LoSSI mode.
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### Transfer modes
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Only interrupt mode is supported.
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### Deprecated warning
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The following appears in the kernel log:
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```
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bcm2708_spi bcm2708_spi.0: master is unqueued, this is deprecated
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```
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### SPI driver latency
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This [thread](http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=19489) discusses latency problems.
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### DMA capable driver
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This is a fork of spi-bcm2708 which enables DMA support for SPI client drivers that support DMA.
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https://github.com/notro/spi-bcm2708 ([wiki](https://github.com/notro/spi-bcm2708/wiki))
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## <a name="troubleshooting"></a>Troubleshooting
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### Loopback test
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This can be used to test SPI send and receive. Put a wire between MOSI and MISO. It does not test CE0 and CE1.
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```
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$ wget https://raw.githubusercontent.com/raspberrypi/linux/rpi-3.10.y/Documentation/spi/spidev_test.c
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$ gcc -o spidev_test spidev_test.c
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$ ./spidev_test -D /dev/spidev0.0
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spi mode: 0
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bits per word: 8
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max speed: 500000 Hz (500 KHz)
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FF FF FF FF FF FF
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40 00 00 00 00 95
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FF FF FF FF FF FF
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FF FF FF FF FF FF
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FF FF FF FF FF FF
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DE AD BE EF BA AD
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F0 0D
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```
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If you get compilation errors, try the latest version instead
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```
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$ wget https://raw.github.com/torvalds/linux/master/Documentation/spi/spidev_test.c
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```

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