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SPI (Linux)

 

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Overview

The Apalis standard provides two SPI interfaces compatible with all modules of the Apalis family.

MXM3 Pin Apalis Signal Name Description
225 SPI1_MOSI SPI Master Output, Slave Input; SPI Data Out (DOUT)
223 SPI1_MISO SPI Master Input, Slave Output; SPI DATA In (DIN)
227 SPI1_CS SPI Chip Select; SPI Enable Signal; SPI Slave Select
221 SPI1_CLK SPI Serial Clock; (SCK)
231 SPI2_MOSI SPI Master Output, Slave Input; SPI Data Out (DOUT)
229 SPI2_MISO SPI Master Input, Slave Output; SPI DATA In (DIN)
233 SPI2_CS SPI Chip Select; SPI Enable Signal; SPI Slave Select
235 SPI2_CLK SPI Serial Clock; (SCK)

The Colibri standard provides one SPI interface compatible with all modules of the Colibri family which historically is called Synchronous Serial Port (SSP).

SODIMM Pin Colibri Signal Name SSP Description SPI Description
86 SSPFRM Synchronous Serial Port Frame SPI Chip Select; SPI Enable Signal; SPI Slave Select
88 SSPSCLK Synchronous Serial Port Clock SPI Serial Clock; (SCK)
90 SSPRXD Synchronous Serial Port Receive SPI Master Input / Slave Output (MISO); SPI DATA In (DIN)
92 SSPTXD Synchronous Serial Port Transmit SPI Master Output / Slave Input (MOSI); SPI Data Out (DOUT)

The Apalis/Colibri standard SPI interfaces are enabled by default. Most modules offer additional (non-standard) SPI interfaces, to enable these device tree or board file aka platform data customization is required.

Modules

Apalis i.MX6

Toradex Name NXP/Freescale Name Device
Apalis SPI1 ECSPI1 SS0 spidev0.0
Apalis SPI2 ECSPI2 SS0 spidev1.0

Apalis T30

Toradex Name NVIDIA Name Device Note
Apalis SPI1 SPI1B CS0 spidev0.0
SPI2B CS0 MCP2515 for Apalis CAN1
SPI4C CS1 MCP2515 for Apalis CAN2
Apalis SPI2 SPI5A CS2 spidev4.2

Apalis TK1

Toradex Name NVIDIA Name Device Note
Apalis SPI1 SPI1A CS0 spidev0.0
SPI2C CS1 Kinetis K20 Companion MCU
SPI2C CS2 spidev1.2 EzPort of Kinetis K20 Companion MCU
Apalis SPI2 SPI4C CS0 spidev3.0

Colibri iMX6

Toradex Name NXP/Freescale Name Device
Colibri SSP ECSPI4 SS1 spidev3.0

By default our BSP uses the MCP2515 SPI CAN controller as available on our Colibri Evaluation board V3.x. In order to use spidev instead one needs to disable (e.g. status = "disabled") the mcp2515 device tree node and enable (e.g. status = "okay") the spidev one and re-generate and deploy the device tree to the target. The specific changes are outlined below:

diff --git a/arch/arm/boot/dts/imx6dl-colibri-eval-v3.dts b/arch/arm/boot/dts/imx6dl-colibri-eval-v3.dts
index acd9031..f8c176d 100644
--- a/arch/arm/boot/dts/imx6dl-colibri-eval-v3.dts
+++ b/arch/arm/boot/dts/imx6dl-colibri-eval-v3.dts
@@ -112,13 +112,13 @@
                interrupt-parent = <&gpio3>;
                interrupts = <27 0x2>;
                spi-max-frequency = <10000000>;
-               status = "okay";
+               status = "disabled";
        };
        spidev0: spidev@1 {
                compatible = "toradex,evalspi";
                reg = <0>;
                spi-max-frequency = <23000000>;
-               status = "disabled";
+               status = "okay";
        };
 };

Colibri iMX7

Toradex Name NXP/Freescale Name Device
Colibri SSP ECSPI3 SS0 spidev2.0

By default our BSP uses the MCP2515 SPI CAN controller as available on our Colibri Evaluation board V3.x. In order to use spidev instead one needs to disable (e.g. status = "disabled") the mcp2515 device tree node and enable (e.g. status = "okay") the spidev one and re-generate and deploy the device tree to the target. The specific changes are outlined below:

diff --git a/arch/arm/boot/dts/imx7-colibri-eval-v3.dtsi b/arch/arm/boot/dts/imx7-colibri-eval-v3.dtsi
index 1db5aa5..28a8bc7 100644
--- a/arch/arm/boot/dts/imx7-colibri-eval-v3.dtsi
+++ b/arch/arm/boot/dts/imx7-colibri-eval-v3.dtsi
@@ -101,14 +101,14 @@
                interrupt-parent = <&gpio5>;
                interrupts = <2 IRQ_TYPE_EDGE_FALLING>;
                spi-max-frequency = <10000000>;
-               status = "okay";
+               status = "disabled";
        };

        spidev0: spidev@0 {
                compatible = "toradex,evalspi";
                reg = <0>;
                spi-max-frequency = <23000000>;
-               status = "disabled";
+               status = "okay";
        };
 };

Colibri T20

Toradex Name NVIDIA Name Device
Colibri SSP SPI4 CS0 spidev3.0

Colibri T30

Toradex Name NVIDIA Name Device
Colibri SSP SPI1A CS0 spidev0.0

Colibri VFxx

Toradex Name NXP/Freescale Name Device
Colibri SSP SPI1 PCS0 spidev1.0

User-space access

SPI access from user-space is provided through the spidev driver which exports device files under /dev. See Documentation/spi/spidev in the kernel sources for more information.

The following shows a read of an ADC082S021 using the sample code in the kernel sources (Documentation/spi/spidev_test.c) using spidev interface:

# ./spidev_test -D /dev/spidevB.C
spi mode: 0
bits per word: 8
max speed: 500000 Hz (500 KHz)

02 E0 00 00 00 00 
00 00 02 E0 02 E0 
02 E0 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 

Kernel-space drivers

The Linux kernel already provides drivers for various SPI devices, hence before writing your own driver checking your Linux kernels configuration options and/or searching through the kernel mailing list is best practice.

If you plan to write a SPI device driver you can use the SPI MCP2515 CAN controller driver as an example. The MCP2515 CAN controller is available and connected to the Colibri SPI port on the v3 series of the Colibri Evaulation Board.

Frequency

Colibri T20

By default the regular SPI interface is configured for 3 MHz operation. The sbc4 clock which is four times the SPI clock is configured for 12 MHz operation with pll_p as its parent. As the divider between its parent and sbc4 only supports 7.1 bits (e.g. 7 bits of mantissa and 1 bit of fraction) and pll_p runs at 216 MHz the lowest possible SPI clock in that configuration lays somewhere around 424 kHz. If a lower clock is desired one has to choose another parent clock like clk_m operating at 13 MHz. This allows SPI clocks as low as 25 kHz. In the Linux kernel sources that configuration can be found here.

As follows some clock tree excerpts for a few use-cases:

DEFAULT spidev configuration

   clk_m                           on     11           13000000
      pll_p                        on     11  x16.6..  216000000
         sbc4                      off    0   18       12000000


MCP2515 CAN configuration

   clk_m                           on     11           13000000
      pll_p                        on     11  x16.6..  216000000
         sbc4                      off    0   5.5      39272727

=> 9.830 MHz measured at SSP_SCLK pin

   clk_m                           on     11           13000000
      pll_p                        on     11  x16.6..  216000000
         sbc4                      off    0   127.5    1694117   

=> 423.6 kHz measured at SSP_SCLK pin

   clk_m                           on     11           13000000
      sbc4                         off    0   65       200000    

=> 50 kHz measured at SSP_SCLK pin