The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
Listing 12-1. Causes an encoder to transmit requested data.
10 REM 8255 mode set: Ports A,C = output, Port B = input
20 XBY(0FC03H)=82H
30 DO
40 INPUT “Enter the decoder’s address (0-15): ”,A
50 INPUT “Enter the data to send (0-15): ”,D
60 REM Write the address and data information to Port A
70 XBY(0FC00H)=(D*10H+A)
80 REM Toggle TE (Port C, bit 7)
90 XBY(0FC03H)=0EH
100 XBY(0FC03H)=0FH
110 WHILE 1=1
120 END
Computer-controlled Receiver
Instead of, or in addition to, computer control of the transmitter, you can also add a computer
interface at the receiver. For example, a data logger might accept data from a remote
transmitter and process the data or store it for later use. Figure 12-7 shows a receiver similar
to Figure 12-2’s, with the manual controls replaced by port bits of an 82(C)55.
The circuit uses different port bits from Figure 12-6’s circuit, so you can connect both a
transmitter and a receiver to one 8255 if you wish. The low nibble of Port C is configured
as an output, and sets A1-A4 on the decoder. As in the previous circuit, A5 is tied high so you
can set the address with 4 bits. Port B is configured as an input, and its bits 4-7 store the
data received at the decoder’s D6-D9. VT is inverted and then connects to the 8052’s INT1
(pin 13). You can use any CMOS inverter.
Listing 12-2 is a BASIC-52 program that sets up the 8255 to receive data at port B, in Mode
0. The program writes an address to the decoder’s address inputs, and also turns off TE (PC.7
in Figure 12-6, to ensure that the encoder on this end (if connected) isn’t transmitting while
the decoder is receiving. The program uses an edge-detecting interrupt to ensure that the
program won’t re-interrupt if VT is still low when the interrupt routine ends.
The main program is a do-nothing loop that waits for an interrupt. When VT goes high,
indicating that a valid transmission has been received, the 8052-BASIC executes an interrupt
routine that reads the data at bits 4-7 of Port B and displays it on the host computer.
Using VT to generate an interrupt is a handy way to detect when new data has arrived, but
you don’t have to use interrupts. If you don’t enable interrupt 1, you can read Port 3’s bit 3
periodically to find out if a new transmission has arrived. Or, you can leave VT unconnected
210 The Microcontroller Idea Book