The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
Basic tests
When you have the circuits built and tested, if you have an oscilloscope or frequency counter,
you can measure the frequency at pin 3 of U3 or pin 3 of U2A. If you’re using U3, adjust
R8 as needed for a 40-kHz output. You can also measure at pin 12 of U1 to verify that its
oscillator is about 1 kilohertz.
To send a test transmission, set A1-A5 identically at U1 and U4, and set D6-D9 to the values
you want to transmit. The schematics show the components set up to transmit the value 1000
to address 00001.
Aim IRED1 so that it points to MOD1’s photodiode window. To begin, place the transmitter
and receiver a few feet apart. To transmit, press S1 momentarily to pulse pin 14 of U1 low.
At U4, LED5 should flash to indicate that a valid transmission was received. At D6-D9, LED4
should be on and LEDs 1-3 should be off, to show that the value 1000 was received.
To change the data to be transmitted, move one or more jumpers or switches at U1’s D6-D9.
Press S1, and LEDs 1-4 should change to match. With these circuits, I was able to receive
data from 12 feet away, with only casual aligning of the transmitter and receiver.
If you weren’t able to measure and adjust U3’s frequency, you can do so now. Jumper pin
12 of U1 to ground to cause the transmitter to continuously transmit. With IRED1 aimed at
MOD1, slowly adjust potentiometer R8 until LED5 lights, continue to adjust until LED5
turns off, and then return R8 to about the middle of the range where LED5 is on. You can
then keep the potentiometer, or replace it with a single resistor that matches the value you
found experimentally.
To add a second receiver, build another circuit identical to Figure 12-2’s, but with the address
inputs set differently. In this way, you can transmit to a selected receiver by changing A1-A5
at the transmitter. Even if a receiver detects a transmission meant for another receiver, it will
ignore it, since the address doesn’t match.
Computer-controlled Transmitter
Figure 12-6 is a transmitter that is similar to 12-1, but with the manual controls replaced by
outputs of an 82(C)55 PPI (from Chapter 6). Port A and bits 4-7 of Port C are configured as
outputs.
The two halves, or 4-bit nibbles, of the 8255’s Port A control the transmitted data and address.
The high nibble controls data inputs D6-D9, and the low nibble controls address inputs A1-A4.
The fifth address input, A5, is tied high, so you can control both the data and address with
one 8-bit port. This reduces the number of decoders you can transmit to from 32 to 16, but
this shouldn’t be a problem in most applications.
208 The Microcontroller Idea Book