#Working with SPI bus under Linux Kernel 2.6.x

##Aria G25

To enable and configure the SPI bus on Aria G25 (or Terra) using the Kernel 2.6.39 
we have to edit a source file inside the Kernel tree:

* [board-sam9x5ek.c](https://github.com/tanzilli/ariag25-linux-2.6.39/blob/ariag25/arch/arm/mach-at91/board-sam9x5ek.c#L151)

The CS0 pin is defined as first element in this struct:

<pre class='prettyprint'>
static struct spi_board_info ariag25_spi_devices[] = {
    {
        .modalias    = "spidev",
        .chip_select    = 0,
        .max_speed_hz    = 10 * 1000 * 1000,
        .bus_num    = 0,
        .mode = SPI_MODE_3,
    },
};
</pre>

To enable the CS1 signal simply add a second element to the struct in that way:

<pre class='prettyprint'>
static struct spi_board_info ariag25_spi_devices[] = {
    {
        .modalias    = "spidev",
        .chip_select    = 0,
        .max_speed_hz    = 10 * 1000 * 1000,
        .bus_num    = 0,
        .mode = SPI_MODE_3,
    },

	{
		.modalias    = "spidev",
		.chip_select    = 1,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},
};
</pre>

To enable the CS2 signal add another element to the struct as shown 
below but disable the **TTY0** peripheral (**/dev/tty1** device). Note that 
on the [Terra-M board](/terra) this line is used by the GPRS modem.

<pre class='prettyprint'>
	{
		.modalias    = "spidev",
		.chip_select    = 2,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},
};
</pre>

To disable the **TTY0** peripheral (**/dev/tty1**) change the line:

<pre class='prettyprint'>
	/* USART0 on /dev/ttyS1. (RX,TX,RTS,CTS) */
	at91_register_uart(AT91SAM9X5_ID_USART0,1,ATMEL_UART_RTS|ATMEL_UART_CTS);
</pre>

in:

<pre class='prettyprint'>
	/* USART0 on /dev/ttyS1. (RX,TX,RTS,CTS) */
	/* at91_register_uart(AT91SAM9X5_ID_USART0,1,ATMEL_UART_RTS|ATMEL_UART_CTS); */
</pre>


To generate a new kernel image and use it on the Aria G25 or Terra board read this article: 

* @article='ariag25_compile_linux_2_6_39'.

##FOX Board G20

To enable and configure the SPI bus on FOX Board G20 using the Kernel 2.6.38 we have to edit a source 
file inside the Kernel tree:

* [board-foxg20.c](https://github.com/tanzilli/foxg20-linux-2.6.38/blob/acme-2.6.38/arch/arm/mach-at91/board-foxg20.c#L128).

After this section:

<pre class='prettyprint'>
static struct spi_board_info foxg20_spi_devices[] = {
#if !defined(CONFIG_MMC_AT91)
	{
		.modalias	= "mtd_dataflash",
		.chip_select	= 1,
		.max_speed_hz	= 15 * 1000 * 1000,
		.bus_num	= 0,
	},
#endif
</pre>

add these lines:

<pre class='prettyprint'>
static struct spi_board_info foxg20_spi_devices[] = {
	// Add CS0

	{
		.modalias    = "spidev",
		.chip_select    = 0,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},

	// Add CS1

	{
		.modalias    = "spidev",
		.chip_select    = 1,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},

	// Add CS2

	{
		.modalias    = "spidev",
		.chip_select    = 2,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},

	// Add CS3
	{
		.modalias    = "spidev",
		.chip_select    = 3,
		.max_speed_hz    = 10 * 1000 * 1000,
		.bus_num    = 1,
		.mode = SPI_MODE_3,
	},
}; 
</pre>

To generate a new kernel image and use it on the FOX Board G20 read this article: 

* @article='compile_linux_2_6_38'.

##SPI signals

As stated before the SPI mode is an important parameter that should be 
trimmed according to your slave device and in accompliance with the modes 
available [here](http://lxr.linux.no/linux+v2.6.38/include/linux/spi/spi.h#L69).

Now follow some graphs of the spidev behaviours captured with an oscilloscope. 
In the following images the cyan trace is the SCLK signal, the yellow trace is MOSI signal and at the end the green trace is n-th CS.

## Spidev on CS0

The trace below was captured using the following Kernel definition:

<pre class='prettyprint'>
    {
        .modalias    = "spidev",
        .chip_select    = 0,
        .max_speed_hz    = 1000000,
        .bus_num    = 1,
        .mode = SPI_MODE_3,
    },
</pre>

and the following command:

<pre class="minicom">
echo spi0 > /dev/spidev1.0
</pre>

Note that the training char in the captured image is a carrier return introduced by echo.

<img src="./CS0.png">

## Spidev on CS1

The trace below was captured using the following Kernel definition:

<pre class='prettyprint'>
    {
        .modalias    = "spidev",
        .chip_select    = 1,
        .max_speed_hz    = 1000000,
        .bus_num    = 1,
        .mode = SPI_MODE_1,
    },
</pre>

and the following command:

<pre class="minicom">
echo test > /dev/spidev1.1* command as spidev writer.
</pre>

Note that using a mode1 the behaviour of clock signal is different.

<img src="./CS1.png">

## Spidev on CS2

The trace below was captured using the following Kernel definition:

<pre class='prettyprint'>
    {
        .modalias    = "spidev",
        .chip_select    = 2,
        .max_speed_hz    = 1000000,
        .bus_num    = 1,
        .mode = SPI_MODE_3 | SPI_LSB_FIRST,
    },
</pre>

and the following command:

<pre class="minicom">
echo -n spi2 > /dev/spidev1.2
</pre>

Note that in this final capture the mode is the binary OR of more than one flag defined inside [spi.h kernel header](http://lxr.linux.no/linux+v2.6.38/include/linux/spi/spi.h#L69).
The captured traces show also that the byte-ordering is inverted due to SPI_LSB_FIRST flag and also that only 4 char are sent on the bus due to the -n option of echo command that supprime the carrier return.

<img src="./CS2.png">