TS-4900 CPU SPI Controller
The CPU has 2 SPI controllers which are accessible through either specific kernel drivers, or userspace using the /dev/spi interface. To utilize SPI, most projects will end up with a customized device tree, so setting up the kernel build environment will be necessary. See the kernel compile guide here for more details.
Open the device tree source file such as arch/arm/boot/dts/imx6qdl-ts4900-reve.dtsi or arch/arm/boot/dts/imx6qdl-ts4900.dtsi, or find the device tree that matches your baseboard. The kernel requires a spidev device be added to the relevant ECSPI controller. For example:
&ecspi2 {
fsl,spi-num-chipselects = <2>;
cs-gpios = <&gpio6 2 0>, <&gpio5 29 0>;
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_ecspi2>;
status = "okay";
serial1: max3100-1@0 {
compatible = "max3100-ts";
reg = <0>;
interrupt-parent = <&gpio1>;
interrupts = <4 IRQ_TYPE_LEVEL_LOW>;
spi-max-frequency = <10000000>;
loopback = <0>;
crystal = <1>;
poll-time = <100>;
};
spidev: spi@1 {
compatible = "spidev";
reg = <1>;
spi-max-frequency = <18181818>;
};
};
In this case ECSPI2 is configured with a spidev at 18.181818MHz which will be available at /dev/spidev1.1 (<bus counting from 0>.<chipselect>). This example adds the spidev device for the offboard chip select. The above example uses GPIO 5 29 which is SPI_2_CS#/CN2_65. The line "reg = <1>;" must be declared to select which chip select in the "cs-gpios" array will be asserted when communicating to your device. After this is configured rebuild the kernel and install your new device tree.
On this board ECSPI1 is used for the boot flash. If this bus is expanded care must be taken to limit trace lengths, and make sure the chip selects are not asserted out of boot. If another device attempts to drive MISO on startup, or the trace lengths are long enough to cause signal integrity issues these will prevent boot.
The SPI max speed is varied between CPU, pins, and if the SPI transaction is read/write:
| Bus | CPU | Read | Write |
|---|---|---|---|
| ECSPI1 | IMX6Q | 40 ns (25.00 MHz) | 15 ns (66.66 MHz) |
| IMX6DL | 43 ns (23.25 MHz) | 15 ns (66.66 MHz) | |
| ECSPI2 | IMX6Q | 55 ns (18.18 MHz) | 15 ns (66.66 MHz) |
| IMX6DL | 43 ns (23.25 MHz) | 15 ns (66.66 MHz) |
See the i.MX6 datasheet for further details on SPI timing such as setup, hold, and propagation delays.
Once you have a /dev/spidev device, you can open this file and use the standard Linux SPI API. For more information see the documentation and sample code: