The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 7
A 1-microfarad capacitor at KBM and 0.1-microfarad capacitor at OSC give a keyboard
debounce of 10 milliseconds and a scan rate of around 600 hertz. Larger values will increase
the debounce time and decrease the scan rate. The 74C922’s data sheet recommends
choosing KBM’s capacitor to be ten times the values of OSC’s capacitor, so be sure to change
both if you change either.
Testing the Keypad
Listing 7-3 tests Figure 4’s circuit. The program waits for a key press and when one occurs,
displays the value of the key on the host computer.
The following paragraphs explain the program in greater detail.
Lines 10-25 are a lookup table that translates the 74C922’s A-D outputs into an ASCII code
corresponding to the key pressed. The table is stored at locations 1FF0h-1FFFh in external
RAM. The lookup table is arranged with the values of the 74C922’s data outputs (0-Fh) in
ascending order. Notice that the key legends (0-F) do not follow in order in the table, because
the legends on the keypad correspond to the data outputs only at keys 4-6 and F.
Lines 30-50 are the main program loop. This does nothing except wait for an interrupt. When
you press a key, the 74C922’s DA output goes high. The falling edge at INT1 then causes the
program to jump to line 100.
Lines 100-110 read the value of the keypress at AD0-AD3 and, use the lookup table to translate
the key press into an ASCII code. For example, if the key labeled “7" is pressed, Y3 goes
low when X1 is scanned, and the 74C922’s AD outputs identify the keypress as 1000 in binary,
or 8 decimal, which the 8052 reads at E000h. Line 100 ANDs AD0-AD7 with 0Fh to clear
bits 4-7, leaving 8, the value of the keypress. In line 110, 8 + 1FF0h = 1FF8h, and the value
stored at 1FF8h in external memory is 55, which is the ASCII code for the numeral 7.
Line 120 causes the character matching the keypress to display on-screen. The program then
returns to the main loop to wait for another key press.
If you have a keypad with different encoding, change the lookup table to match its layout.
Customizing the Interface
Listing 7-3 does little more than test the interface, but you can use the general idea in a
specific project. You can also assign special functions to individual keys. In an EPROM
programmer, these might be Select device, Program, Verify, and so on. If the functions aren’t
labeled on the keys, you can describe them in an on-screen menu: Press 1 to select device;
press 2 to program; and so on. Then, when a keypress is detected, instead of just displaying
the value of the key, your program would branch to a subroutine that corresponds to the
122 The Microcontroller Idea Book