Hardware Reference
In-Depth Information
If yours fails to respond to safe mode, it may be due to a manufacturing error. See this
message:
In that thread, it is suggested that you check the following:
$ vcgencmd otp_dump | grep 30:
30:00000002
If you see the value 2, it means that the firmware thinks this is a Rev 1.0 board
(even though it may be a Rev 2.0). When that applies, it will not support the safe mode
sequence. Newer Rev 2.0 Pis do not have this issue.
When safe mode is invoked by the jumper, the config.txt file is ignored except for the
avoid_safe_mode parameter. Additionally, this mode overrides the kernel command line,
and kernel_emergency.img is loaded. If this file is unavailable, kernel.img is used instead.
The intent of this feature is to permit the user to overcome configuration problems
without having to edit the SD card on another machine in order to make a correction. The
booted emergency kernel is a BusyBox image with /boot mounted so that adjustments
can be made. Additionally, the /dev/mmcblk0p2 root file system partition can be fixed up
or mounted if necessary.
Logic Levels
The logic level used for GPIO pins is 3.3 V and is
not
tolerant of 5 V TTL logic. The
Raspberry Pi PCB is designed to be plugged into PCB extension cards or otherwise
carefully interfaced to 3 V logic. Input voltage parameters V
IL
and V
IH
are described in
Chapter 10. This feature of the Pi makes it an interesting case study as we interface it to
the outside world.
GPIO Configuration at Reset
The Raspberry Pi GPIO pins can be configured by software control to be input or output,
to have pull-up or pull-down resistors, or to assume some specialized peripheral
function. After reset, only GPIO 14 and 15 are assigned a special function (UART). After
boot up, however, software can even reconfigure the UART pins as required.
When a GPIO pin is configured for output, there is a limited amount of current
that it can drive (source or sink). By default, each P1 GPIO is configured to use an 8 mA
driver, when the pin is configured as an output. Chapter 10 has more information on the
software control of this.
■
Note
raspbian 1-Wire bus is GpiO 4 (GpCLK0) pin p1-07.
