ARM embedded system serial port expansion

ARM embedded systems often encounter the need for multiple serial ports, but the number of native UARTs in ARM chip systems is limited, so it is necessary to expand more interfaces through other high-speed buses. This article uses Toradex 's Apalis i.MX6D/Q ARM computer module based on NXP i.MX6D/6Q processor to expand 8 serial ports through the EXAR solution under the Linux system.

The Apalis i. MX6D/Q module can support up to 5 UART serial port outputs. Compatible with high-speed TIA/EIA-232F (up to 5Mbit/s). Supports 7, 8 or 9 (for RS485) bits of data, 1 or 2 stop bits. UART1 is a full-featured serial port, and the rest of the serial ports can also support RTS and CTS signals.

In Linux systems, a serial port is usually reserved for application debugging and development and system upgrades. Although functions such as SSH can also be used for remote network access and system debugging, for embedded products, the information when the system starts, especially the Uboot startup information, can help with function debugging and problem location. However, this information can only be output from the serial port. When updating the Linux BSP of the Toradex module, it is also necessary to do so in Uboot.

The remaining 4 serial ports of the Apalis i.MX6Q/D module can not only use TTL level to directly control the corresponding peripherals, but also be expanded to RS232/RS485/RS422 commonly used industrial control ports. For the demand for more serial ports, there are currently multiple solutions to achieve serial port expansion, such as through USB, SPI, Memory Bus, I2C and PCIe buses. Memory Bus and PCIe have higher real-time performance than other buses, and more serial ports can be expanded on the same interface. For applications with high serial port quantity and data real-time performance, these two expansion solutions can be preferred. At the same time, Memory Bus and PCIe are high-speed signal buses, which require some special considerations in PCB wiring. Toradex also provides free PCB design guidance for this purpose. Next, we will introduce how to use EXAR's XR17V358 solution based on the PCIe bus to expand 8 serial ports.

1). XR17V358 solution introduction and driver download

The 8 extended serial ports of XR17V358 all support RTS/CTS or DTR/DSR flow control functions. Each serial port has a 256-byte FIFO, independent clock output, supports half-duplex RS485, and has a maximum transmission speed of 25 Mbps. XR17V358 uses PCIe 2.0 Gen 1 to connect to Apalis i.MX6Q/D to ensure high-speed real-time data transmission. EXAR currently provides Windows and Linux drivers for XR17V358. Here we use its latest Linux driver and port it to the Apalis i.MX6 platform. Driver source code download address http://www.exar.com/common/content/document.ashx?id=20121

2). Configure the compilation environment

Before compiling, you also need to download the Linux kernel and cross-compilation tools for Apalis i.MX6.

a). Download the Linux kernel for Apalis i.MX6

$ git clone -b toradex_imx_3.14.28_1.0.0_ga-next git://git.toradex.com/linux-toradex.git

 

b). Download cross-compilation tools

$ wget http://releases.linaro.org/14.11/components/toolchain/binaries/arm-linux-gnueabihf/gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf.tar.xz

$ tar xvf gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf.tar.xz

$ ln -s gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf gcc-linaro

$ export ARCH=arm

$ export PATH=~/gcc-linaro/bin/:$PATH

$ export CROSS_COMPILE=arm-linux-gnueabihf-

Note: The above path needs to correspond to the directory where the cross-compilation tool is actually unpacked.

 

c). Compile the Linux kernel and provide the necessary configuration files for XR17V358.

$ make apalis_imx6_defconfig

$ make -j4 uImage LOADADDR=10008000

 

d). Compile XR17V358 driver

// Edit the Makefile file and point KERNEL_SRC to the directory where the Linux kernel is located

KERNEL_SRC = /home/ban/Toradex/oe-core-tegra/LinuxKernel/v2.5/mx6/toradex_imx_3.14.28_1.0.0_ga-next/linux-toradex

// Save and run the make command to compile, making sure the ARCH, PATH, and CROSS_COMPILE parameters mentioned above are still valid.

$ make

// After successful compilation, the kernel module file xr17v35x.ko for the ARM processor will be generated

$ file xr17v35x.ko

xr17v35x.ko: ELF 32-bit LSB relocatable, ARM, EABI5 version 1 (SYSV), BuildID[sha1]=399121b7862105b185e24b45 ba3522f14158295e, not stripped

 

e). Install the driver

Copy xr17v35x.ko to the Apalis i.MX6 module and install

root@apalis-imx6:~# insmod xr17v35x.ko

[ 151.156648] Exar PCIe (XR17V35x) serial driver Revision: 2.0

 

root@apalis-imx6:~# lspci

00:00.0 PCI bridge: Device 16c3:abcd (rev 01)

01:00.0 PCI bridge: PLX Technology, Inc. PEX 8605 PCI Express 4-port Gen2 Switch (rev aa)

02:01.0 PCI bridge: PLX Technology, Inc. PEX 8605 PCI Express 4-port Gen2 Switch (rev aa)

02:02.0 PCI bridge: PLX Technology, Inc. PEX 8605 PCI Express 4-port Gen2 Switch (rev aa)

02:03.0 PCI bridge: PLX Technology, Inc. PEX 8605 PCI Express 4-port Gen2 Switch (rev aa)

03:00.0 Serial controller: Exar Corp. Device 0358 (rev 03)

 

The corresponding serial port device files ttyXR0 to ttyXR7 appear in the /dev directory.

root@apalis-imx6:/dev# ls

autofs network_latency tty18 tty60

block network_throughput tty19 tty61

bus null tty2 tty62

char port tty20 tty63

console ppp tty21 tty7

cpu_dma_latency ptmx tty22 tty8

cuse ptp0 tty23 tty9

disk pts tty24 ttyXR0

dri ram0 tty25 ttyXR1

fb ram1 tty26 ttyXR2

fb0 ram10 tty27 ttyXR3

fb1 ram11 tty28 ttyXR4

fb2 ram12 tty29 ttyXR5

fb3 ram13 tty3 ttyXR6

fd ram14 tty30 ttyXR7

 

f). Set the baud rate

root@apalis-imx6:~# stty -F /dev/ttyXR0 115200

After the driver is loaded, it can be used normally in Linux like other serial ports.