PWM with the 80C517/537

Description

The 80C517/80C537 is a 8051 derivate. It's also a superset of the 515/535.
It offers the following features wich are listed below:

Suitable for...

I used the 537 to control a DC/DC motor via a MOSFET based amplifier. I also used it to control a amplifier usualy used for RC models. The CCU is just right for problems like that. It's working interrupt based. You just have to set up the parameters (frequency, pulse width, ...) and the CCU is doing the rest. See example (description/code) below.

Using the CCU

This is a PWM example controlling a RC amp like used for RC models (I did it for the A.R.T. project). I messured the output of the RC receiver. The output had a frequency/period of 40[Hz]/0.025[s]. I found out that the amp stops the motor at a pulse width of about 0.0102[s]. Maximum rpm is reached at a pulse width of 0.0105[s].

Adjusting the Timer

The CCU is based on timer #2. The clock of my 537 is 12M[Hz]. So I adjusted the prescaler to divide the clock input by 12. No we have a time base which is 1M[Hz]. The timer starts with the reloadvalue. If the timer has an overflow (0xffff to 0x0000) the interrupt #5 is triggered. If you select auto reload mode the timer is loaded with the reloadvalue again and you have a periodic signal (which is exactly what we need). What reload value do wee need to reach a period of 0.025[s]? The time is determined by the difference of the reload value to 0xffff. So you get the expression (0xffff - reload) / timebase = period. We know the period and want to know the reload value. So we rearrange the expression and get reload = -(period*timebase - 0xffff). With our values we have reload = -(0.025 * 10^6 - 0xffff) = 0x9e57.
The table is showing the configuration of the 537's special function registers (SFR).

The SFRs are set as follows:
CRC = 0x9e57 - Reloadvalue for timer, determines the period length/frequency
T2CON = 0x11 - Timer #2 autoreload (periodic), compare mode

Adjusting the Pulse Width

The 537's CCU has four channels. We just need two for our differential steering. So we just have to configer the SFRs CC1 and CC2 and enable those two channels (CCEN register).
Each channel is associated with a SFR (CC) and a output port line. If the timer reaches the value in CC (CC1-CC4) the output is going high (1). It is going low (0) again at timer overflow. The pulse width of the high level is given by CC-0xffff.
The interrupt service routine (ISR) is the right place to update the current pulse width. Set the width outside of the ISR. The ISR will make it become valid at the non critical time period.

This is the association between CC register and port: CC1 - P1.1; CC2 - P1.2; CC3 - P1.3 and CC4 - P1-4.

Don't forget: CCEN = 0x28

Example Code

unsigned int pw_left;   /* Pulsewidth for left and right motor. */
unsigned int pw_right;

/*
 *  After the initialization done in main() the CCU  produces a periodic
 *  pulse (width 0,0102s). The current pulse width is adjusted during
 *  the interrupt service routine.
 */
_interrupt 5 _using 0 void ccu_isr( void )
{
       ET2 = 0; /* Disable timer #2 */
       CCEN &= 0xd7; /* Compare disabled */
       CC1 = pw_left; /* Get pulse width from variables */
       CC2 = pw_right; /* Use as the new compare values */
       CCEN = 0x28; /* Compare enabled */
       TF2 = 0; /* reset the timer #2 Overflow Request */
       ET2 = 1; /* enable timer #2 */
}

/*
 *  Controlling two DC motors by PWM.
 */
void main()
{
    /*
     *  Initial pulsewidth for both motors.
     */
    pw_left = 0xfc00;
    pw_right = 0xfc00;
    /*
     *  Set the 537's CCU to a periodic signal (40Hz) and set up comparemode
     */
    EAL = 0; /* All interrupts disabled. */
    CRC = 0x9e57; /* Timer #2 reloadvalue for 40Hz (0,025s) */
    CC1 = 0xfc00; /* Adjust pulse width for both channels, so that */
    CC2 = 0xfc00; /* initially the motor is turned off (CC = Comparevalue) */ 
    CCEN = 0x28; /* Compare enabel for Chanal 1 and 2 (P1.1, P1.2) */
    T2CON = 0x11; /* Timer #2 autoreloadmode, comparemode */
    ET2 = 1; /* Activate timer #2 */
    EAL = 1; /* All interrupt enabled. */
    /*
     * Mainprogram
     */
    while( 1 )
    {
        ...
        /*
         *  Adjust the pulsewidth for both motors by modifying
         *  pw_left and pw_right.
         */
        ...
    }
}

Related Sites

Welcome to www.6502.org! - www.6502.org/

Related Literature

Michael Fredershausen: Microcontroller Technik in der Praxis
Siemens: SAB 80C517/537 8-Bit CMOS Single-Chip Microcontroller

Related Docs

Datasheet miniMODUL Docking Board - mmBaseHandbook.pdf (287k bytes)
Datasheet miniMODUL-537 - mm537Handbook.pdf (202k bytes)

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this page was updated: 27.05.00 10:18:47
holger@zahnleiter.org

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