Файл: Microcontrollers in Practice (Mitescu M., 2005).pdf

The rest of the modules are included in an endless program loop. TIMER.ASM defines a set of software timers, as described in Chap. 9. INPUT.ASM reads the analog input lines AN0–AN3, connected to the temperature sensor and three external potentiometers, and updates the variables TS, TM, KP and TI. For scaling purposes, the 8-bit value read from AN2 is truncated to the four most significant bits, so that KP is limited to take values in the range [0–15]. Similarly, TI is reduced to take values in the range [0–63]. The digital input lines PA0–PA6 are read to update the variables SWSTATUS, which encodes the status of the rotary switch, and QSTART, a Boolean variable set to $FF when the START button is pressed, and cleared by pressing the STOP button. The subroutine CERR (Compute Error) is called here and updates the variable ERR – a 16-bit signed integer, defined as the difference (TS − TM).

The module INTEGRAL.ASM approximates the term:

Ti

1 × e(t)dt . Ti

0

Using the notation Ei = e(i × T0), then the integral can be approximated by the sum:

This means that the integral term in the expression (14.3) can be approximated by the average of the last N values of the variable ERR = TS − TM. The number N of samples considered for computing the integral is determined by the variable TI. T0 is a constant, equal to 100 ms, implemented by means of a software timer.

The values of the samples Ei are stored in a buffer IBUF. The size of IBUF must be dimensioned so that it can store the maximum value of samples, as indicated by the variable TI. Since TI can have one of 64 possible values, and the error ERR is a 16-bit integer, then IBUF must be128 bytes in length.

Two additional bytes are reserved for a pointer in IBUF, named XIBUF. IBUF is organized as a circular buffer. Each time the timer that defines T0 expires, the computed value of ERR is stored in the buffer at the location indicated by XIBUF, and then the pointer is incremented by 2. When the end of buffer is reached, the pointer is reloaded with the starting address of IBUF.

Only the last TI values in IBUF are used to compute the average. The result is stored in the variable INTEGRAL This is used by the module CONTROL.ASM, which computes VOUT = KP (ERR + INTEGRAL). VOUT is adjusted to be an 8-bit value in the range [0–255].

182 14 PI Temperature Controller

The computed value of VOUT is used in OUTPUT.ASM to determine the moment at wich to generate the pulse to open the triac that drives the heater. This is done in the interrupt service routine of TIC3. When an edge on IC3 (PA0) occurs, the TIC3 interrupt service routine prepares a value for TOC1, based on the value of VOUT, then enables the TOC1 interrupt.

To avoid time-consuming calculations in the interrupt service routine, VOUT is used as an offset in a table (TOCTAB) that contains for each possible value of VOUT, a 16-bit value that is added to the current TCNT and written to TOC1. These cause a TOC1 interrupt to be generated after a number of E cycles equal to the value extracted from TOCTAB.

The TOC1 interrupt service routine simply generates a pulse on PA7, then disables further TOC1 interrupts. A new interrupt will only be generated when an edge is sensed on IC3 and QSTART = $FF.

Here is what the interrupt service routines for TIC3 and TOC1 look like:

The module DISPLAY.ASM contains the software routines needed to control the four-digit seven-segments display unit described in Appendix A14.