Hardware Reference
In-Depth Information
The 22 k W pull-up resistor in the schematic diagram is necessary to pull the Out line
up to V CC level, when no 38 kHz beam is seen. (The datasheet block diagram shows the
output as a CMOS totem-pole output, but the pull-up suggests open-drain configuration
instead.) When the device sees a carrier for a minimum burst of 300 μs, it pulls the Out
line low. This line remains driven low as long as the carrier signal is detected. As soon as
the carrier is removed for 300 μs or more, the line is pulled high again by the resistor.
Out
Description
High
No 38 kHz carrier seen
Low
38 kHz carrier is detected
Wired as shown, the Raspberry Pi will be able to see the effect of the carrier being
turned on and off, many times per second, as it receives remote-control bursts of IR light.
Software
This is where I apologize in advance. No matter which brand of TV or remote control
is supported by the software in this chapter, most people will own something different .
However, if you own a relatively recently produced Samsung TV, the software might just
work for you out of the box. The software presented in this chapter was developed for the
remote control of a Samsung plasma HDTV (Model series 50A400).
If the software doesn't work for you as is, then consider yourself blessed. When
you dig into the program and make it work for your remote, you'll come away from the
experience knowing much more than when you started.
Signal Components
Here's where it gets fun. While most manufacturers have agreed on the 38 kHz carrier
frequency, they haven't agreed on how the signaling works. Most protocols work on the
principle of turning bursts of IR on and off, but how that encodes a “key” differs widely.
An informative website ( www.techdesign.be/projects/011/011_waves.htm )
documents a few of the common IR waveforms. 54 The one we're interested in is the
Samsung entry, listed as protocol number 8.
Table 5-1 summarizes the technical aspects of the waveforms shown at the website.
All times shown are in milliseconds.
 
 
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