Embedded Linux Development Board
Embedded Linux Development Board
Demo
Overview
The motivation for this project was to create from scratch a complete embedded linux development platform for learning and experimentation.
Building a development board from scratch is not trivial. Going through the process myself highlighted the following key considerations:
- Stackup layout in order to minimize Electromagnetic Interference (EMI) and ground loop effects
- Power management considerations: Modern ICs may require different power supply input voltages which need to be brought up in a specific order for correct operation
- Trace clearance recommendations must be adhered to to avoid EMI interfernce
- Trace impedences to match application specifications (e.g. USB2.0 requires 45 $$\Omega$$ differential impedence)
- Delay matching - high speed applications require signals to arrive within a certain tolerance to avoid data corruption
- Passive component selection is more than simply selecting the nominal value. One must consider real world performance. For example capacitors operating with a forward DC bias sees a decrease in overall capacitance.
- Availability of components must be checked to ensure they still supported and can be procured easily
The key features of the board I designed and manufactured include:
- Microchip SAM9X60 ARM926EJ-S based MPU running at 600MHz
- 64MB DDR2-800 SDRAM
- mirco SD card with ESD protection
- 100Mbit Ethernet
- 2 USB Type A ports with over-current and ESD protection
- Support for a 24 bit parallel LCD touch display (specifically targeting the Newhaven’s NHD-4.3-800480CF-ASXP-CTP)
- ARM EmbeddedICE JTAG debug interface
Design screenshots
Software Design
On the software side, the board runs a custom embedded Linux distribution I built using the Yocto project. Main components of the distribution include the AT91Bootstrap bootloader, u-boot acting as the third stage bootloader, Linux kernel, custom devicetree, systemd init system, busybox utilities and the Atom package manager.
Device Tree
Yocto project layers
layer path priority
========================================================================================================
core /home/joash/strawberry_os/poky/meta 5
yocto /home/joash/strawberry_os/poky/meta-poky 5
yoctobsp /home/joash/strawberry_os/poky/meta-yocto-bsp 5
openembedded-layer /home/joash/strawberry_os/poky/../meta-openembedded/meta-oe 5
networking-layer /home/joash/strawberry_os/poky/../meta-openembedded/meta-networking 5
webserver /home/joash/strawberry_os/poky/../meta-openembedded/meta-webserver 5
meta-python /home/joash/strawberry_os/poky/../meta-openembedded/meta-python 5
meta-initramfs /home/joash/strawberry_os/poky/../meta-openembedded/meta-initramfs 5
atmel /home/joash/strawberry_os/poky/../meta-atmel 10
multimedia-layer /home/joash/strawberry_os/poky/../meta-openembedded/meta-multimedia 5
meta-arm /home/joash/strawberry_os/poky/../meta-arm/meta-arm 5
arm-toolchain /home/joash/strawberry_os/poky/../meta-arm/meta-arm-toolchain 5
meta-strawberry_sam /home/joash/strawberry_os/meta-strawberry_sam 6Custom layer file structure
meta-strawberry_sam/
├── COPYING.MIT
├── README
├── conf
│ ├── layer.conf
│ └── machine
│ ├── include
│ │ ├── at91sam9.inc
│ │ ├── bootloaders.inc
│ │ └── sam9x60.inc
│ └── sam9x60custom.conf
├── recipes-bsp
│ ├── at91bootstrap
│ │ ├── at91bootstrap_%.bbappend
│ │ └── files
│ │ ├── defconfig
│ │ └── sam9x60custom_linux_dt_defconfig
│ └── u-boot
│ ├── files
│ │ ├── envs
│ │ │ └── sam9x60custom.txt
│ │ ├── sam9x60custom_defconfig
│ │ └── strawberry_sam.dts
│ └── u-boot-mchp_2024.07.bbappend
├── recipes-example
│ └── example
│ └── example_0.1.bb
└── recipes-kernel
└── linux
├── files
│ ├── at91-strawberry_sam.dts
│ ├── newhaven-43-800480-timings.patch
│ └── sam9x60custom_config.patch
├── linux-mchp_%.bbappend
└── temp.txt