B+ HAT ID EEPROM FORMAT SPECIFICATION
CURRENTLY THIS SPEC IS PRELIMINARY AND THEREFORE STILL LIKELEY TO CHANGE
Note that there are software tools for creation of EEPROM images and for flashing / dumping images from attached HATs.
HEADER <- EEPROM header (Required) ATOM1 <- Vendor info atom (Required) ATOM2 <- GPIO map atom (Required) ATOM3 <- DT blob atom (Required for compliance with the HAT specification) ... ATOMn
EEPROM Header Structure
Bytes Field 4 signature signature: 0x52, 0x2D, 0x50, 0x69 ("R-Pi" in ASCII) 1 version EEPROM data format version (0x00 reserved, 0x01 = first version) 1 reserved set to 0 2 numatoms total atoms in EEPROM 4 eeplen total length in bytes of all eeprom data (including this header)
Bytes Field 2 type atom type 2 count incrementing atom count 4 dlen length in bytes of data+CRC N data N bytes, N = dlen-2 2 crc16 CRC-16-CCITT of entire atom (type, count, dlen, data)
0x0000 = invalid 0x0001 = vendor info 0x0002 = GPIO map 0x0003 = Linux device tree blob 0x0004 = manufacturer custom data 0x0005-0xfffe = reserved for future use 0xffff = invalid
Vendor info atom data (type=0x0001):
Note that the UUID is mandatory and must be filled in correctly according to RFC 4122 (every HAT can then be uniquely identified). It protects against the case where a user accidentally stacks 2 identical HATs on top of each other - this error case is only detectable if the EEPROM data in each is different. The UUID is also useful for manufacturers as a per-board 'serial number'.
Bytes Field 16 uuid UUID (unique for every single board ever made) 2 pid product ID 2 pver product version 1 vslen vendor string length (bytes) 1 pslen product string length (bytes) X vstr ASCII vendor string e.g. "ACME Technology Company" Y pstr ASCII product string e.g. "Special Sensor Board"
GPIO map atom data (type=0x0002):
GPIO map for bank 0 GPIO on 40W B+ header.
NOTE GPIO number refers to BCM2835 GPIO number and NOT J8 pin number!
Bytes Field 1 bank_drive bank drive strength/slew/hysteresis, BCM2835 can only set per bank, not per IO Bits in byte: [3:0] drive 0=leave at default, 1-8=drive*2mA, 9-15=reserved [5:4] slew 0=leave at default, 1=slew rate limiting, 2=no slew limiting, 3=reserved [7:6] hysteresis 0=leave at default, 1=hysteresis disabled, 2=hysteresis enabled, 3=reserved 1 power [1:0] back_power 0=board does not back power Pi 1=board back powers and can supply up to 1.3A to the Pi 2=board back powers and can supply up to 2A to the Pi 3=reserved If back_power=2 high current USB mode is automatically enabled. [7:2] reserved set to 0 28 1 byte per IO pin Bits in each byte: [2:0] func_sel GPIO function as per FSEL GPIO register field in BCM2835 datasheet [4:3] reserved set to 0 [6:5] pulltype 0=leave at default setting, 1=pullup, 2=pulldown, 3=no pull [ 7] is_used 1=board uses this pin, 0=not connected and therefore not used
Device tree atom data (type=0x0003):
Binary data (DT blob fragment for board hardware).
For more information on the Devicetree atom contents, see the Devicetree Guide.