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PWM AND ANALOG OUTPUT SECTION 12

PAGE 12-1

INTRODUCTION

PWM AND ANALOG OUTPUT SECTION 12

The RPC-220 has two program mable PW M outputs.

These outputs are the source for the two analog outputs.

Thus, when using an analog output, its corresponding

PWM outpu t may not be used. PWM frequen cy is

programm able for a pproxim ately 162 Hz to 43Khz. Duty

cycle is variable from 0 to 100% in 256 steps. The

frequency is set for both PW M outpu t channels w hile

each channel had independently pr ogram mable duty

cycles.

Analog output takes the PWM signal and filters it.

Output voltage range is programmable from 0 to about

5V in 256 steps. It is effectively an 8 bit D-A.

PROGRAMMING PWM

PWM output signals come directly from the CPU at port

J3. J3-22 is designated as PWM0 while J3-20 is PWM1.

There is no buffering. Drive levels are TTL compatible.

Lines must be buffered to directly drive opto modules.

Repetition frequency for both outputs is given by:

fpwm = 43369/(1+ PWMP)

Where:

PWM P is the prescaler frequency control

address 0xfe. Its range is 0 to 255.

The duty cycle (in percent) for both outputs is given by:

Dcy = 100 * (1 - (PWM n/255))

Where:

PWMn is from 0 to 255. PWM0 is at special

function register address 0xfc and PWM1 is at

0xfd.

100% duty cycle is defined as always high while 0 % is

always low.

Quick PWM Program ming E xample

PWM outputs are at J3-22 for PWM0 and J3-20 for

PWM 1. To pr ogram them, use monitor command ' MS'

to modify the registers. For example type in the

following command.

MS fc

Address 0xfc is PWM 0 duty cycle register.

The board responds with

00FC 00

Type in a 40. This makes channel 0 a mostly high pulse.

Enter the data shown below. This programs PWM

outputs for 4.8 Khz repetition rate pulses. View the

outputs at J3-22 for PWM0 and J3-20 for PWM1.

00FC 00 40 00FD 00 c0 00FE FF 08 <cr>

Address 0xfd is PWM 1 duty cycle register and address

0xfe determines both channels r epetition rate

(frequency).

Example program PWM.C under directory PWM

generates a ram p at the analog outputs by continuously

varying the duty cycle.

ANALOG OUTPUTS

Output voltage is propor tional to the PW M channel' s

duty cycle. Thus, the output voltage changes in steps of

1/255, or approximately 20 mv/ step. This makes the

analog output act like an 8 bit D-A converter. Accuracy

depends upon the 5V supply. The analog outputs are

proportional to the PWM duty cycle.

Using on card regulators, maximum output voltage can

be 0.25 from the ideal 5.0V. Thus, the output can be as

much as 5% off.

When analog output is used as part of a system w ith

feedback, absolute accu racy is ge nerally no t critical.

This is because the output changes according to demand

requirements. When the output is used as a reference

(i.e. 2. 5V = 1/2 speed) and there is no feedback, then

accuracy can suffer. In situations wh ere acc uracy is

important, it is better to use a card with accurate analog

output (such as the RPC-210), or adjust the output by

reading the input.

The output may be " adjusted" by bringing the outputs to

one of the A-D's on board. (The A-D uses a fixed

reference so accuracy is higher. ) Simply read the analog

output and adjust the duty cycle as necessary to bring the

voltage to the correct level.

Response Time

Since the analog outputs are der ived from digital pulses,

there ar e a couple of things you have to take into

account. First, there is some ripple on the output. The

amount of ripple depends upon the PWM frequency.

Generally, the higher the frequency, the better. The

CPU PWM P register should be program med for 0x10 or

less. Lower PW M frequencies increase ripple.

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Remote-processing RPC-220 Manual de usuario Pagina 38

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