The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
causes brief, random pulses to appear at VOUT even when an IRED isn’t transmitting to the
module. But as we’ll see, these random pulses are rejected by the decoder chip, which looks
for a specific pulse pattern to identify the transmissions intended for it.
To reduce false triggers on ambient light, you can ground MOD1’s case by soldering a wire
from pin 3 to the case. You can also add more optical filtering, though for this application,
it shouldn’t be necessary. Photographic film is a good, inexpensive filter that passes infrared
and blocks visible light. Cover the photodiode’s window with an exposed, developed scrap
of color-print negative film, or an unexposed, developed scrap of (positive) color-slide film.
The signal at pin 1 of MOD1 is essentially the same as U1’s DATA OUT in Figure 12-1, but
inverted. U5A inverts MOD1’s output so that pin 9 of U4 matches U1’s DATA OUT. You can
substitute just about any CMOS inverter.
The decoder chip. U4 requires timing components to match U1’s oscillator frequency. R9
and C7 set the timing that discriminates between narrow and wide received pulses. R10 and
C8 set the timing that detects the end of an encoded word and the end of a transmission.
Figure 12-2 shows Motorola’s formulas and recommendations for choosing these values.
U4 has five address lines (A1-A5), which must match A1-A5 on U1. As with U1, the inputs
are trinary, and may be logic high, logic low, or open. For testing, you can set these with
jumpers or switches.
When MOD1 transmits, U4 examines the incoming bits at its pin 9. If the five address bits
received match U4’s address, U4 stores the next four bits and compares them to the previous
four data bits received. If the data bits don’t match, D6-D9 don’t change. If the data bits do
match, the receiver latches the new data to D6-D9 and brings VT (pin 11) high to indicate that
a valid transmission was received.
The receiver doesn’t latch D6-D9 until it receives the same data twice in a row. This
complements the behavior of U1, which automatically sends each transmission twice.
Requiring the receiver to see the same data twice prevents the receiver from accepting data
that was garbled in transmitting. The only way that an error can slip through is if the address
transmits correctly both times, and the data contains the same error twice in a row—if a
transmitted 0 shows up as a 1 at the receiver, for example. The chances of this are small,
especially since the 40-kHz modulation adds another layer of rejection of unwanted signals.
The data at D6-D9 remains until it is replaced by new received data. VT remains high until
an error is detected or until there is no input for four data-bit times (32 milliseconds at 1
kHz).
Figure 12-2 shows LEDs at D6-D9 and VT for monitoring these outputs during testing.
Current-limiting resistors aren’t required, since U4 sources only about 5 milliamperes
206 The Microcontroller Idea Book