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 (bank 0) map atom (Required)
ATOM3 <- DT blob atom (Required for compliance with the HAT specification)
...
ATOMn-1
ATOMn <- GPIO (bank 1) map atom (Optional, only available on CM1, CM3, CM3+ and CM4S)
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 of entire atom (type, count, dlen, data)
0x0000 = invalid
0x0001 = vendor info
0x0002 = GPIO (bank 0) map
0x0003 = Linux device tree blob
0x0004 = manufacturer custom data
0x0005 = GPIO (bank 1) map
0x0006-0xfffe = reserved for future use
0xffff = invalid
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 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
Binary data (the name or contents of a .dtbo overlay, for board hardware).
For more information on the Device Tree atom contents, see the Device Tree Guide.
GPIO map for bank 1 GPIOs (28-45) only available on CM1, CM3, CM3+ and CM4S. This must be the last atom.
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 reserved
[7:0] reserved set to 0
18 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