REF: BBONEBLK_SRM
BeagleBone Black System
Reference Manual
Rev A5.2
Page 85 of 108
be used as well. Make sure the device you select supports 16 bit addressing. The part
package used is at the discretion of the cape designer.
8.2.1
EEPROM Address
In order for each cape to have a unique address, a board ID scheme is used that sets the
address to be different depending on the setting of the dipswitch or jumpers on the capes.
A two position dipswitch or jumpers is used to set the address pins of the EEPROM.
It is the responsibility of the user to set the proper address for each board and the position
in the stack that the board occupies has nothing to do with which board gets first choice
on the usage of the expansion bus signals. The process for making that determination and
resolving conflicts is left up to the SW and, as of this moment in time, this method is a
something of a mystery due t the new Device Tree methodology introduced in the 3.8
kernel.
Address line A2 is always tied high. This sets the allowable address range for the
expansion cards to
0x54
to
0x57
. All other I2C addresses can be used by the user in the
design of their capes. But, these addresses must not be used other than for the board
EEPROM information. This also allows for the inclusion of EEPROM devices on the
cape if needed without interfering with this EEPROM. It requires that A2 be grounded on
the EEPROM not used for cape identification.
8.2.2
I2C Bus
The EEPROMs on each expansion board are connected to I2C2 on connector P9 pins 19
and 20. For this reason I2C2 must always be left connected and should not be changed by
SW to remove it from the expansion header pin mux settings. If this is done, then the
system will be unable to detect the capes.
The I2C signals require pullup resistors. Each board must have a 5.6K resistor on these
signals. With four capes installed this will be an effective resistance of 1.4K if all capes
were installed and all the resistors used were exactly 5.6K. As more capes are added the
resistance is reduced to overcome capacitance added to the signals. When no capes are
installed the internal pullup resistors must be activated inside the processor to prevent
I2C timeouts on the I2C bus.
The I2C2 bus may also be used by capes for other functions such as I/O expansion or
other I2C compatible devices that do not share the same address as the cape EEPROM.
8.2.3
EEPROM Write Protect
