THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

EMBEDDED MULTITASKING

The Easy Way
by Charles Kosina

Start ý An Example ý State Machine Control ý Keyboard Scanning ý Receive Serial Data ý LCD Module ý Transmit Data ý Debugging Techniques ý Other Options ý Sources and PDF

KEYBOARD SCANNING

Figure 2 shows the state machine for the keyboard. Only three states are needed. State 0 waits for a keystroke in the 4 ý 4 matrix. The decoded character is then stored in a memory location and a flag is set. This flag is also used by other tasks. State 1 then waits for the key to be released. Finally, in state 3 there is a de-bounce delay of about 10 ms before going back to state 0.

Figure 2ýThe Keyboard Scan State Machine interrogates a 4 ý 4 keyboard matrix. It is activated 60 times per second.

Each state machine has a control block. Listing 4 shows how this is done for the 8051. There are a few important things to note. Each subroutine in the jump table must have only one entry and exit (standard programming practice). And, keep the subroutine short because there are other tasks waiting for their share of CPU time.

Obviously there is overhead involved. About seven instructions are executed every time a state is entered and exited. The number of instructions executed within a state can vary enormously, from three to 700. So, depending on your programming style, the particular chip that you use, and how much processing is done within each state, the overhead could range from 10% to 50%.

One thing to watch for is that the ajmp instruction range is only 4 KB. If your program exceeds this you may get an "out of page" error. To fix this, replace all the jump table ajmp instructions with ljmp instructions and the rl a with add a,#3 because there are 3 bytes per instruction.

Note that the first two instructions allow the task to be disabled by setting the kb_disable bit. This prevents a potential race condition where a keystroke could alter the data that is being accessed by another task.

PREVIOUS • NEXT