Datasheet

Compute Module Datasheet

Supply Minimum Requirement Unit

VBAT (CM1) 2000

VBAT (CM3,3L) 3500

3V3 250 mA

1V8 250 mA

VDAC 25 mA

GPIO0-27 VDD 50

GPIO28-45 VDD 50

SDX VDD 50

Recommended minimum. Actual power drawn is very dependent on use-case

Each GPIO can supply up to 16mA, aggregate current per bank must not exceed 50mA

Table 8: Mimimum Power Supply Requirements

8 Booting

The 4GB eMMC Flash device on CM3 is directly connected to the primary BCM2837 SD/eMMC inter-

face. These connections are not accessible on the module pins. On CM3L this SD interface is available

on the SDX pins.

When initially powered on, or after the RUN pin has been held low and then released, the BCM2837

will try to access the primary SD/eMMC interface. It will then look for a ﬁle called bootcode.bin on the

primary partition (which must be FAT) to start booting the system. If it cannot access the SD/eMMC

device or the boot code cannot be found, it will fall back to waiting for boot code to be written to it over

USB; in other words, its USB port is in slave mode waiting to accept boot code from a suitable host.

A USB boot tool is available on Github which allows a host PC running Linux to write the BCM2837

boot code over USB to the module. That boot code then runs and provides access to the SD/eMMC as a

USB mass storage device, which can then be read and written using the host PC. Note that a Raspberry Pi

can be used as the host machine. For those using Windows a precompiled and packeged tool is available.

For more information see here.

The Compute Module has a pin called EMMC DISABLE N which when shorted to GND will disable

the SD/eMMC interface (by physically disconnecting the SD CMD pin), forcing BCM2837 to boot from

USB. Note that when the eMMC is disabled in this way, it takes a couple of seconds from powering up

for the processor to stop attempting to talk to the SD/eMMC device and fall back to booting from USB.

Note that once booted over USB, BCM2837 needs to re-enable the SD/eMMC device (by releasing

EMMC DISABLE N) to allow access to it as mass storage. It expects to be able to do this by driving

the EMMC EN N 1V8 pin LOW, which at boot is initially an input with a pull up to 1V8. If an end user

wishes to add the ability to access the SD/eMMC over USB in their product, similar circuitry to that

used on the Compute Module IO Board to enable/disable the USB boot and SD/eMMC must be used;

that is, EMMC DISABLE N pulled low via MOSFET(s) and released again by MOSFET, with the gate

controlled by EMMC EN N 1V8. Ensure you use MOSFETs suitable for switching at 1.8V (i.e. use a

device with gate threshold voltage, Vt, suitable for 1.8V switching).

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