Hardware Reference
In-Depth Information
JTAG Ports
Between the composite video out and the 3.5-mm audio out, you will find two rows of pin headers labeled P2 and P3.
•
P2 is the JTAG, or Joint Test Action Group, interface for the Broadcom SoC. There is not much
you can do with this JTAG port at the moment.
•
Likewise, the P3 header is the JTAG port for the SMSC LAN9512 chip. It is also of not much use
right now.
The JTAG interfaces are used to initially program the SoC chip and the SMSC chip on the board. The manufacturer
can also use the JTAG to test the hardware at the time of its manufacture. If the Raspberry Pi did give us access to the
JTAG interface you could re-flash the Broadcom chipset with your own firmware to run your own software.
Audio Output
Wow, another somewhat boring output. It's the 3.5-mm stereo audio socket. Just like with the HDMI and composite
outputs, you can only output audio to one output at a time, such as HDMI or the 3.5 mm. You can use HDMI video
with the audio disabled: this will enable you to use the 3.5-mm audio with the HDMI output.
By default if you use HDMI, the audio will go via HDMI; if you want HDMI video and 3.5-mm audio, set this in
your
config.txt
file using
hdmi-drive=1
. Sound-quality wise, the HDMI output will provide you with a better output
but if you don't have that then you can make do with the 3.5-mm stereo output.
LEDs
Lastly, on the edge of the Raspberry Pi you will find five LEDs. While not inputs, they do have an output: their ability to
emit light. This output tells us a few key status points about the Pi's operation state and the link state for the Ethernet
on the Model B only. In Table
1-3
we can see the modes for the LEDs.
Table 1-3.
LED States on the Raspberry Pi
LED and Led Color
Description
D5 Green
System okay/SD card access
D6 Red
Power okay, 3.3 V
D7 Green
Full duplex; half duplex if the LED is off
D8 Green
Link activity for the LAN
That's it for the inputs and outputs on the Raspberry Pi. There are a few miscellaneous chips on the back but
none of these are important to discuss.
Pi Brains
What's this in Figure
1-2
? It's the ARM11 core block diagram that makes up part of the SoC. This is only one part of
the Broadcom BCM2835 SoC and it is officially called the ARM1176JZF-S core. Don't feel too bad if Figure
1-2
looks
like just a bunch of shapes thrown onto the page or if anything in this next section seems way too technical. As you
go though each chapter and learn more about the Raspberry Pi and its hardware, this section will start to make more
sense. Give it a read over now but don't worry if you just don't get it. Please do come back to this section when you're
involved with the projects; you will be surprised at how much more it makes sense.
