[Nexus 7 Q&A, Help & Troubleshooting] bftb0:
I don't know if all this info was ever collected into one post; if not, here it is. The times given are approximate times to hold the buttons continuously.

[size=+1]Starting from Device OFF Condition:[/size]

Power (2-3 seconds) -> Normal OS Boot**

Vol-Up + Power (1-3 seconds) -> APX Mode

Vol-Down + Power (6-8 seconds) -> Bootloader/fastboot Mode (keep both buttons pressed until you see the droid lying on his back).

[size=+1]Starting from APX Mode:[/size]

Power (10-15 seconds) -> Normal OS Boot**

[size=+1]Starting from booted or boot-looping OS:[/size]

Power (10-15 seconds) -> Normal OS Boot**

** Bootloader Interrupt: In any of the cases where you hold the power button down to try to break out of the current operating mode, if you press the Vol-Down button right after you first see the black and white screen with the Google logo, the tablet will proceed immediately to bootloader/fastboot mode, rather than booting the normal OS. You need to be ready though - you only get about 1 or 2 seconds to do this! (Thanks to RMXO for pointing this out) Note that this means you can get to the bootloader directly from any other mode!

This also explains why, when attempting a cold start using Vol-Down + Power, if you accidentally let go of the vol-down button when the screen first lights up, the Nexus 7 will boot to the normal OS. In this case, pressing Vol-Down initially is actually superfluous - what makes the device go into bootloader/fastboot mode is the Vol-Down press immediately following the google splash screen - there actually is no unique "Go to fastboot from a cold start" method.

[size=+1]Starting from Bootloader/Fastboot Mode:[/size]

Follow the instructions on-screen.

[size=+1]Starting from Stock Recovery Splash Screen:[/size]

Power + Vol-Up simultaneous press -> gets you to the hidden stock recovery Menu.

For me, pressing Vol-Up first and then Power does not seem to work here - they seem to need to be pressed simultaneously.

[size=+1]Starting from Custom Recovery:[/size]

Follow the instructions on-screen.

[size=+1]Screen Shot in Booted OS[/size]

Press Power + Vol-Down Simultaneously & hold for about 1 second. The simultaneous part seems to be important, as well as holding them down together for the one second or so. A fine example of a simply horrid UI implementation. (Google, can your Grandma perform this trick?)

[size=+1]Safe Mode Boot[/size]

From a cold start boot (press & release Power button), wait 2 seconds after the B&W Google (bootloader) splash screen appears, then press and hold Vol-Down button until the normal OS boot is fully completed (lock screen or home screen appears)

[size=+1]Discussion.[/size]

1) These results can depend on the bootloader version; I tested all of the above on the v4.13 bootloader under the following scenarios: (a) unlocked bootloader and custom recovery, (b) unlocked bootloader and the JOP40D stock recovery, and (c) locked bootloader and JOP40D stock recovery. I noted no differences in button-press responses due to these different configurations.

2) Note that in APX mode, there are absolutely no indications on the screen that the device is actually turned on. The ONLY way to see this is to plug it in to a computer, and you will see/hear the device insertion. I am able to enter APX mode without being plugged in, and then afterwards plug into the PC, or cold start it into APX mode plugged in to the PC at cold-boot. If I unplug the tablet in APX mode and then attempt a re-insert, Win 8 is unhappy with the device.

3) If you are stuck in a bootloop pay special attention to the Bootloader Interrupt note above - this is how you get out of a bootloop and into the bootloader or recovery!

4) The JOP40D Stock Recovery has a two minute (120 sec) timer running - if you do not progress to the (hidden) menu within this time, an automatic boot to the regular OS will occur.

[size=+1]Bonus Info: USB Device IDs in each mode[/size]

Bootloader/Fastboot Mode:

USB\VID_18D1&PID_4E40&REV_0000 (Android Phone -> Android Bootloader Interface)

USB\VID_18D1&PID_4E40

Stock OS, ADB Debugging Off, PTP Storage

USB\VID_18D1&PID_4E43&REV_9999 (Portable Devices -> Nexus 7 ) ["PTP"]

USB\VID_18D1&PID_4E43

Stock OS, ADB Debugging Off, MTP Storage

USB\VID_18D1&PID_4E41&REV_9999 (Android Phone -> Android ADB Interface)

USB\VID_18D1&PID_4E41

(Note: MTP Seems to need to have ADB Debugging enabled - 4.2.1/JOP40D)

Stock OS, ADB Debugging ON:

USB\VID_18D1&PID_4E42&REV_9999&MI_01 (Android Phone -> Android Composite ADB Int)

USB\VID_18D1&PID_4E42&MI_01

(when in MTP storage mode)

USB\VID_18D1&PID_4E42&REV_9999&MI_00 (Portable Devices -> Nexus 7 ) ["MTP"]

USB\VID_18D1&PID_4E42&MI_00

(when in PTP storage mode)

USB\VID_18D1&PID_4E44&REV_9999&MI_00 (Portable Devices -> Nexus 7 ) ["PTP"]

USB\VID_18D1&PID_4E44&MI_00

APX Mode:

USB\VID_0955&PID_7330&REV_0103 (Other Devices -> APX)

USB\VID_0955&PID_7330

Custom Recovery - TWRP 2.3.3.0

USB\VID_18D1&PID_D001&REV_9999 (Android Phone -> ?varies? )

USB\VID_18D1&PID_D001

If you are aware of other escape sequences using the physical buttons to navigate through different device modes, or differences in these behaviors in earlier bootloader revisions, let me know.

Revision History

---------------------------------------------------------------------------------

2013-02-04 Initial Revision

2013-02-08 Added USB Bus Hardware IDs for PTP mode

2013-03-29 Added Safe Mode