The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Inputs and Outputs
Listing 6-2. Sets or clears a bit at output port E400h.
5 REM variable A contains the last data written to 0E400h
10 INPUT “Enter a bit to set or clear (0-7) :”,X
20 INPUT “Enter 1 to set, 0 to clear :”,Y
30 IF Y=1 THEN A=A.OR.2**X
40 IF Y=0 THEN A=A.AND.0FFH-2**X
50 XBY(0E400H)=A
60 END
Listing 6-1 is a program that reads the buffer and displays the value of each bit. The program
is similar to Listing 3-1, which displays the bit values at Port 1.
Line 20 finds the logic state of bit 0 by logically ANDing the byte with a mask byte that is
all 0’s except for bit 0: 00000001. The result is 1 if bit 0 is 1, and 0 if bit 0 is 0. Lines 30-90
are similar, except that each time a different bit in the mask byte is 1. In each case, the
program divides the result by 2 raised to the power of the bit number. Since (2**0 equals 1,
line 20 leaves out this step.) Each PRINT statement shows the logic state of one of the bits.
At the output port, if you want to change just one bit in the byte, you have to know the
current value of the byte. The simplest way to accomplish this is to save the last value you
wrote. Or, you could wire an input buffer at the same address, with each input bit connecting
to the corresponding output bit, and read the input when you need to know the current value.
Listing 6-2 prompts you for a bit to set and clear, then does so. It assumes that you’ve stored
the current value of E400h in variable A.
To set a bit, line 30 logically ORs the current value with a mask byte that is all 0s except for
the bit to be set. For example, to set bit 4, the mask byte is 0001 0000, or 10h, which leaves
bits 0-3 and 5-7 unchanged, but forces bit 4 to be 1.
To clear a bit, line 40 logically ANDs the byte with a mask byte that is all 1s except for the
bit or bits to be cleared. For example, to clear bit 3, the mask byte is 1111 0111, or F7h. The
result is that bits 0-2 and 4-7 are unchanged, but bit 3 must be 0.
Line 50 writes the new byte to the port.
The Microcontroller Idea Book 97