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PIC18F8722 FAMILY

FIGURE 18-1: SIMPLIFIED BLOCK DIAGRAM OF THE ENHANCED PWM MODULE

Duty Cycle Registers

CCP1CON<5:4>

P1M1<1:0>

CCP1M<3:0>

 

 

 

 

 

2

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CCPR1L

 

 

 

 

 

 

 

 

 

ECCP1/P1A

ECCP1/P1A

 

 

 

 

 

TRISx<x>

CCPR1H (Slave)

 

 

 

P1B

P1B

 

 

 

 

Comparator

R

Q

Output

TRISx<x>

Controller

 

 

 

 

 

 

 

 

 

 

P1C

P1C

TMR2

(Note 1)

 

 

 

S

 

 

TRISx<x>

 

 

 

 

 

 

 

 

 

Comparator

 

 

 

P1D

P1D

Clear Timer,

 

 

 

 

 

 

 

 

TRISx<x>

 

set ECCP1 pin and

 

 

 

 

 

 

 

 

PR2

latch D.C.

 

 

 

 

 

 

 

ECCP1DEL

 

 

 

 

 

 

Note: The 8-bit TMR2 register is concatenated with the 2-bit internal Q clock, or 2 bits of the prescaler, to create the 10-bit time base.

18.4.2PWM DUTY CYCLE

The PWM duty cycle is specified by writing to the CCPR1L register and to the CCP1CON<5:4> bits. Up to 10-bit resolution is available. The CCPR1L contains the eight MSbs and the CCP1CON<5:4> contains the two LSbs. This 10-bit value is represented by CCPR1L:CCP1CON<5:4>. The PWM duty cycle is calculated by the equation:

EQUATION 18-2:

PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) • TOSC • (TMR2 Prescale Value)

CCPR1L and CCP1CON<5:4> can be written to at any time but the duty cycle value is not copied into CCPR1H until a match between PR2 and TMR2 occurs (i.e., the period is complete). In PWM mode, CCPR1H is a read-only register.

The CCPR1H register and a 2-bit internal latch are used to double-buffer the PWM duty cycle. This double-buffering is essential for glitchless PWM operation. When the CCPR1H and 2-bit latch match TMR2, concatenated with an internal 2-bit Q clock or two bits of the TMR2 prescaler, the ECCP1 pin is cleared. The maximum PWM resolution (bits) for a given PWM frequency is given by the equation:

EQUATION 18-3:

 

log(

FOSC

)

 

PWM Resolution (max) =

FPWM

bits

 

 

 

 

log(2)

 

 

 

 

 

Note: If the PWM duty cycle value is longer than the PWM period, the ECCP1 pin will not be cleared.

TABLE 18-4: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS AT 40 MHz

PWM Frequency

2.44 kHz

9.77 kHz

39.06 kHz

156.25 kHz

312.50 kHz

416.67 kHz

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Timer Prescaler (1, 4, 16)

16

4

1

1

1

1

 

 

 

 

 

 

 

PR2 Value

FFh

FFh

FFh

3Fh

1Fh

17h

 

 

 

 

 

 

 

Maximum Resolution (bits)

10

10

10

8

7

6.58

 

 

 

 

 

 

 

2004 Microchip Technology Inc.

Preliminary

DS39646B-page 193

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