When you want to design a shield for Raspberry Pi, you have to follow the HAT standard. This standards describes the form factor of the shield to ensure future compatibility. It also describes the configuration solution for the board based on a flash containing necessary information. The Hat standard link gives all the detailed specification to design a such shield. In this post I’ll describe my experience of designing such shield and you will find the basic elements I build and now sharing with you.

Design the board

The first step is to design the board following the HAT standard ; the next picture is an extract from the documentation on github, to start your design, please always refer to github here (https://github.com/raspberrypi/hats/blob/master/hat-board-mechanical.pdf) has it could be subject to change. The kicad resource I’m sharing are based on this extract

Based on this, you can find the Kicad empty PCB corresponding to this and looking like this with the eprom already routed

The following github contains the full kicad project with shematics, component libs …

https://github.com/disk91/hat-design

Select the chips

One of the main issue I had was to find a 2×20 PIN header corresponding to the requirement : a height between 10 and 12 mm ; one of the corresponding model is the 3M 4P01-2A02-DA that can be found here

• http://www.digikey.fr/product-search/fr?x=0&y=0&lang=fr&site=fr&KeyWords=3M10619-ND (4.65€ HT / unit)
• http://www.mouser.fr/ProductDetail/3M-Electronic-Solutions-Division/4P01-2A02-DA/?qs=yDj%2FBb4wmPEDM94jFjRvdQ%3D%3D

But, I recommend to use these other one that fits better because you have to take into account the 2mm of plastic on the male header :

Another compatible header is : PPPC202LFBN-RC (3€)
from Sullins Connector Solutions – less expensive, but ensure the size is right before ordering – my reading of the documentation is only 8.5mm high. But with the 2 mm of the header plastic on the board, it is 10.5mm. You can source here :

• http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=S7123-ND

Or this one from 3M: 929975-01-20 (2.6€)

• http://www.mouser.fr/Search/ProductDetail.aspx?R=929975-01-20virtualkey51750000virtualkey517-929975-01-20

Then you can use a 11mm spacer between the Pi and the shield like this one :

• http://au.mouser.com/Search/ProductDetail.aspx?R=M1258-2545-SSvirtualkey68520000virtualkey761-M1258-2545-SS

If you have better sourcing, please tell me in the comment as this one is expensive !

Then you have to select a flash, I choose a CAT24C32WIG from On Semiconductor, following Hat spec recommendations, you can find here

• http://www.digikey.fr/product-detail/fr/CAT24C32WI-G/CAT24C32WI-GOS-ND/2061011 (0,6€)
• http://www.mouser.fr/ProductDetail/ON-Semiconductor/CAT24C32WI-G/?qs=sGAEpiMZZMuVhdAcoizlRUcTcLSfHcN3Sw6b4QhAlcE%3d (0,5€)

Ensure you have the last firmware

as root execute the firmware update

# rpi-update

Then reboot and check firmware version

# /opt/vc/bin/vcgencmd version

you should be version >= 27 Mar 2015 with kernel >= 3.18.10

 

Program the Serial Flash

The Serial flash contains the description of the shield and the configuration of the different pin. Take a look on the Hat Github to get the details on how it works. To create your flash, you can do like this:

Get the github hats tools on your raspberry Pi

# git clone https://github.com/raspberrypi/hats.git
# cd hats/eepromutils

Then you can edit the file eeprom_settings.txt file with your content ; in blue in the file, what I changed to match my card

########################################################################
# Vendor info

# 128 bit UUID. If left at zero eepmake tool will auto-generate
# RFC 4122 compliant UUID
product_uuid 00000000-0000-0000-0000-000000000000

# 16 bit product id
product_id 0x0001

# 16 bit product version
product_ver 0x0001

# ASCII vendor string (max 255 characters)
vendor "Disk91"

# ASCII product string (max 255 characters)
product "SigFox TD1204 Shield"

The next part is about GPIO configuration : in my shield, my configuration is the following one :

<Header GPIO>  |  <Header PIN> | <Board use> | <Type>
GPIO27         |       13      |    LED      | Output - Low
GPIO22         |       15      |    LED      | Output - High
GPIO18         |       12      |    LED      | Output - High
TX             |        8      |    Serial   | Output
RX             |       10      |    Serial   | Input
GPIO23         |       16      |    GPIO     | Unconnected
GPIO24         |       18      |    GPIO     | Unconnected
GPIO25         |       25      |    GPIO     | Unconnected

In the eeprom file we can configure this like

########################################################################
# GPIO pins, uncomment for GPIOs used on board
# Options for FUNCTION: INPUT, OUTPUT, ALT0-ALT5
# Options for PULL: DEFAULT, UP, DOWN, NONE
# NB GPIO0 and GPIO1 are reserved for ID EEPROM so cannot be set

# GPIO FUNCTION PULL
# ---- -------- ----
#setgpio 2 INPUT DEFAULT
#setgpio 3 INPUT DEFAULT
#setgpio 4 INPUT DEFAULT
#setgpio 5 INPUT DEFAULT
#setgpio 6 INPUT DEFAULT
#setgpio 7 INPUT DEFAULT
#setgpio 8 INPUT DEFAULT
#setgpio 9 INPUT DEFAULT
#setgpio 10 INPUT DEFAULT
#setgpio 11 INPUT DEFAULT
#setgpio 12 INPUT DEFAULT
#setgpio 13 INPUT DEFAULT
#setgpio 14 INPUT DEFAULT
#setgpio 15 INPUT DEFAULT
#setgpio 16 INPUT DEFAULT
#setgpio 17 INPUT DEFAULT
setgpio 18 OUTPUT UP
#setgpio 19 INPUT DEFAULT
#setgpio 20 INPUT DEFAULT
#setgpio 21 INPUT DEFAULT
setgpio 22 OUTPUT UP
setgpio 23 INPUT UP
setgpio 24 INPUT UP
setgpio 25 INPUT UP
#setgpio 26 INPUT DEFAULT
setgpio 27 OUTPUT DOWN

I assume Serial Port is configured another way by the system.

This configuration file made, you have to compile it with the eepmake tool.

# first step is to compile eepmake tool itself
> make eepmake
# then execute it
./eepmake eeprom_settings.txt eeprom_settings.eep

Now, before flashing the EEPROM, you must place a jumper on the shield flash header to allow writing into the flash. Then you can flash the board.

Before being able to write the eeprom, we must activate the I2C device ; since device-tree implementation is seems to be unactivated by default.

Edit the file /boot/config.txt and add the following line : dtparam=i2c_vc=on

Then reboot.

Once done, you should be able to run

./eepflash.sh -w -f=./eeprom_settings.eep -t=24c32

Now, if you reboot and have the right firmware version, you should get en entry in the /proc/device-tree/hat directory where you will get the eeprom information in the different files.

You can remove the dtparam setting in the config.txt file

Create a device-tree file

It seems that actually HAT serial configuration is not yet ready, so to have the device correctly configure a device tree file have to be provided. To make this, you have to start by creating a dts (source file) then compile it into a dtb (binary file) and add it to the /boot/overlay directory

To get the compiler install it:

# apt-get install device-tree-compiler

See later … I’ll complete this post later when getting more time & information about this part. Come back 🙂