The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 6
Output Examples
Figure 6-4 shows some basic outputs that you can connect to U14:
(A) Outputs can directly drive any TTL or HCTMOS logic input powered at 5 volts.
(B) To interface to 5-volt HCMOS or 4000-series CMOS devices, add a pull-up resistor to
ensure that high outputs are greater than 3.5 volts.
(C) To translate an output to a lower voltage, use a 74HC4050 buffer or 74HC4049 inverter
powered at the lower voltage.
(D) To translate an output to a higher voltage, use a 74LS26 NAND or similar high-voltage,
open-collector gate, with a pullup resistor to the higher voltage.
(E) For high-current outputs, you can use Texas Instruments’ 7545X series of peripheral
drivers. The 75452 NAND gate can sink up to 300 milliamperes at 0.5V.
(F) U14 can directly drive an LED. Since the 74LS374 can sink more current than it can
source, connect the LED so that it turns on when the output is low. With a 220-ohm
current-limiting resistor, the LED’s forward current is around 13 milliamperes.
(G) As with inputs, an optocoupler is another way to interface different voltages, and to
electrically isolate an output from the microcontroller circuit.
Reading and Controlling Individual Bits
Unlike assembly language for the 8052, BASIC-52 has no logical operators for setting and
clearing individual bits in a byte. But you can use the .AND. and .OR. operators to
accomplish the same thing.
Listing 6-1. Displays the value of each bit at input port E000h.
10 A=XBY(0E000H)
20 PRINT “Bit 0 = ”,(A.AND.1)
30 PRINT “Bit 1 = ”,(A.AND.2)/2
40 PRINT “Bit 2 = ”,(A.AND.4)/4
50 PRINT “Bit 3 = ”,(A.AND.8)/8
60 PRINT “Bit 4 = ”,(A.AND.10H)/10H
70 PRINT “Bit 5 = ”,(A.AND.20H)/20H
80 PRINT “Bit 6 = ”,(A.AND.40H)/40H
90 PRINT “Bit 7 = ”,(A.AND.80H)/80H
100 END
96 The Microcontroller Idea Book