Page 256 - Mechatronics with Experiments

242 MECHATRONICS
PIC 18F4431 Microcontroller The PIC 18F4431 version of the PIC 18Fxxxx
family microcontrollers has a quadrature encoder interface at the following pins for an
incremental encoder interface:
RA2, RA3, RA4 for Index (INDX), Channel A (QEA), Channel B (QEB), respec-

tively.
It also supports 8 channels of PWM output via the following pins:
RB3:0, pins 0 through 3, can be configured for PWM0, PWM1, PWM2, PWM3,

respectively.
RB4 and RB5 pin can be configured for PWM5 and PWM4 output signals,
respectively.
RD6 and RD7 pins can be configured for PWM6 and PWM7 output.

Interrupts on-change-of-state for pins RB pins 4 through 7 (RB7:4) are effective only
if these pins are configured as input. If any of these pins is configured for output, the
interrupt for that pin is disabled.
Timers on PIC 18F4431 are TIMER0, TIMER1, TIMER2, and TIMER5. The timer
control register for each timer (T0CON, T1CON, T2CON, T5CON) is an 8-bit register
which is used to configure (control) the operation of the timer. The bits allows us to:
enable/disable timer mode or counter mode, 8-bit or 16-bit mode, clock source (internal or
external), pre-scaler/post-scaler values. Timer 2 and Timer 5 have PR2 and PR5 re-registers
which define the maximum value to which the timer is to count, then roll-over and generate
interrupt. Timer 1 and CCP1 compare configurations that can be used to trigger a periodic
“ADC conversion start” trigger to the ADC module. Timer 2 can be used as the source to
define the period (base frequency) of the PWM output pin. Other timers generate interrupt
when the counting from 00h to FFh is complete and overflow to 00h occurs. For counter
operation, an external source should be selected. In this mode, pre and post-scaling values
are used. In timer mode, pre and post-scaler values are ignored and timers are incremented
every instruction cycle. For each timer, if the interrupt is enabled via the corresponding
interrupt enabled bit on the corresponding timer control register, when a timer interrupt
occurs, the corresponding timer interrupt flag is set.
Motion feedback module (MFM) has a quadrature encoder interface (EQI), which
is used to interface to an incremental encoder for position feedback. Once the position
feedback is available, the microcontroller can perform closed loop position control in any
actuated motion control system, that is electric or electro-hydraulic or pneumatic. The QEI
interface has three pins: QEA, QEB, INDX which are to be connected to the A, B, and
Index channels of an incremental encoder (the Index channel of an incremental encoder is
sometimes referred as Index C or Z). Using the pulses on Channel A and B, and the phase
relationships between them, the module keeps track of position information using a 16-bit
up/down counter register. An INDX pin can also be connected to another external digital
input other than the Encoder’s Index channel, that is a registration sensor (see Chapter 10
for registration applications and possible use of these).
The following registers are used in the configuration and use of the MFM (QEI)
module:

QEICON 8-bit configuration register
POSCNT 16-bit up/down counter: position register

MAXCNT 16-bit maximum count register