Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

194 The Quintessential PIC Microcontroller

Program 7.6 Incrementing a packed-BCD byte with maximum value of 99.

; ************************************************************

; ************************************************************

simply binary incremented in situ using Indirect addressing. It is then corrected as described. The subroutine assumes that the pointed-to datum is already in a packed-BCD format on entry; it does not convert a natural binary byte to BCD.

Self-assessment questions

7.1Rewrite Programs 7.1 and 7.2 to deal with a packing quantity of one gross (144). The count is to be kept in packed BCD (Hundreds and Tens:Units) which can be used by the background software to display the can tally.

7.2What changes to Example 7.1 would you have to make to allow for a maximum value in the oven of 1000?

7.3Based on Fig. 7.1 design an ISR to perform the following tasks:

•Copy the 16-bit count into two GPRs labelled TEMP_H and TEMP_L.

•Deduct from the previous count reading located in LAST_COUNT_H and LAST_COUNT_L and place the di erence in DIFFERENCE_H and

DIFFERENCE_L.

•Update the previous count with the new count.

•Set a GPR labelled NEW to a non-zero value to signal the background software that a new reading is available. The background routine will clear NEW when it has processed the data.

7.4The speed of a rotating shaft can be measured by using a coded disk to generate a pulse on each angular advance of 10◦, which can be used to interrupt a PIC. If the top speed is 20,000 revolutions per minute, what is the absolute maximum duration of the ISR in this

196 The Quintessential PIC Microcontroller

Based on this hardware, the software must implement the following task list:

•Background routine

1.Zero Ji y count and New flag.

2.Pulse the sounder.

3.Wait until New flag is non zero.

4.Display reading.

5.Repeat forever.

•Foreground routine.

1.IF oscillator THEN increment Ji y count.

2.IF receiver THEN set New flag to non-zero to tell background program that the Ji y count is the final value.

3.Repeat until neither is active.

4.Return

Code the foreground ISR tasked above using a GPR as a flag labelled NEW to tell the background program that the echo has returned and to read the Ji y count as the required value. Use Program 7.3 as your model.

7.6It is proposed to increase the range of the digital echo sounder to 10 meters and resolution to 1 mm. What change in the hardware and software would be required?

7.7The system in SAQ 7.6 has been built and tested. However, readings seem to shift slowly with time. Oscillator drift is suspected but has been proven to be stable. Thinking laterally, one student wonders if the speed of sound varies with atmospheric conditions. After some research he arrives at the formula for temperature dependence as:

Vt = V0 1 + 273∆t

where V0 is the propagation velocity at 20◦C and Vt is the velocity at a temperature of t. How much change in temperature ∆t will there be to cause an error of 1 mm with the sounder measuring at its maximum range?

CHAPTER 8

Assembly language

We have now been writing programs with gay abandon since Chapter 3. For clarity these listings have been written in a human-readable form. Thus instructions have been represented as a short mnemonic, such as return instead of 00000000001000b; the file registers similarly have names, such as INTCON; lines have been labelled and comments attached. Such symbolic representations are only for human consumption. The MCU knows nothing beyond the binary codes making up operation codes and data, such as shown on page 45.

With the help of the device’s instruction set, see Appendix A, it is possible to translate from the human-readable symbolic form to machinereadable binary. This is not particularly di cult for a device such as a PIC that has a reduced set of instructions (RISC) and few address modes. However, it is slow and tedious, especially where programs of a significant length are being coded. Furthermore, it is error prone and di cult to maintain whenever there are changes to be made.

Computers are good at doing boring things quickly and accurately; and translating from symbolic to machine code definitely falls into this category. Here we will briefly look at the various software packages that aid in this translation process.

After reading this chapter you will:

•Know what assembly-level language is and how it relates to machine code.

•Appreciate the advantages of a symbolic representation over machinereadable code.

•Understand the function of the assembler.

•Understand the di erence between absolute and relocatable assembly.

•Understand the role of a linker.

•Appreciate the process involved in translating and locating an assemblylevel language program to absolute machine code.

•Understand the structure of a machine-code file and the role of the loader program.

•Understand the role of a simulator.

200 The Quintessential PIC Microcontroller

Program 8.1 Absolute assembly-level code for our square-root module.

;************************************************************

;* FUNCTION: Calculates the square root of a 16-bit integer *

;************************************************************

;Local declarations

cblock

I:2, COUNT ; Magic number hi:lo byte & loop count endc

end

Tells the assembler that this is the end of the source code. equ

Associates a value to a symbol. For instance the assembler replaces the name STATUS by the value 3 anywhere it appears in an instruction operand. Normally used for Special-Purpose registers (SPRs) and bits within file registers.