beaglebone-black - Flipbook - Page 84
BeagleBone Black
Table 8.2 – continued from previous page
PIN
PROC
SIGNAL
MODE
25
20
21
U7
V9
U9
MMC1_DAT0
MMC1_CMD
MMC1_CLK
1
2
2
If using these pins, several things need to be kept in mind when doing so:
• On the eMMC device, these signals are inputs and outputs.
• The eMMC device will add a load onto these pins.
• When used for other functions, the eMMC cannot be used. This means you must boot from the microSD
slot.
• If using these pins, you need to put the eMMC into reset. This requires that the eMMC be accessible from
the processor in order to set the eMMC to accept the eMMC pins.
• DO NOT drive the eMMC pins until the eMMC has been put into reset. This means that if you choose to
use these pins, they must not drive any signal until enabled via Software. This requires a bu昀昀er or some
other form of hold o昀昀 function enabled by a GPIO pin on the expansion header.
On power up, the eMMC is NOT reset. If you hold the Boot button down, this will force a boot from the microSD.
This is not convenient when a cape is plugged into the board. There are two solutions to this issue:
1. Wipe the eMMC clean. This will cause the board to default to microSD boot. If you want to use the eMMC
later, it can be reprogrammed. 2. You can also tie LCD_DATA2 low on the cape during boot. This will be the
same as if you were holding the boot button. However, in order to prevent unforeseen issues, you need to gate
this signal with RESET, when the data is sampled. After set goes high, the signal should be removed from the
pin.
BEFORE the SW reinitializes the pins, it MUST put the eMMC in reset. This is done by taking eMMC_RSTn
(GPIO1_20) LOW after the eMMC has been put into a mode to enable the reset line. This pin does not connect
to the expansion header and is accessible only on the board.
DO NOT automatically drive any con昀氀icting pins until the SW enables it. This puts the SW in control to ensure
that the eMMC is in reset before the signals are used from the cape. You can use a GPIO pin for this. No, we
will not designate a pin for this function. It will be determined on a cape by cape basis by the designer of the
respective cape.
8.1.3 EEPROM
Each cape must have its own EEPROM containing information that will allow the SW to identify the board and to
con昀椀gure the expansion headers pins as needed. The one exception is proto boards intended for prototyping.
They may or may not have an EEPROM on them. An EEPROM is required for all capes sold in order for them
operate correctly when plugged into the BeagleBone Black.
The address of the EEPROM will be set via either jumpers or a dipswitch on each expansion board. Figure 61
below is the design of the EEPROM circuit.
The EEPROM used is the same one as is used on the BeagleBone and the BeagleBone Black, a CAT24C256. The
CAT24C256 is a 256 kb Serial CMOS EEPROM, internally organized as 32,768 words of 8 bits each. It features
a 64-byte page write bu昀昀er and supports the Standard (100 kHz), Fast (400 kHz) and Fast-Plus (1 MHz) I2C
protocol.
The addressing of this device requires two bytes for the address which is not used on smaller size EEPROMs,
which only require only one byte. Other compatible devices may 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.
8.1. BeagleBone Black Cape Compatibility
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