PIC16F684 datasheet(85/192 Pages) MICROCHIP

85 / 192 page

DS41202F-page 83

PIC16F684

11.3.1

PWM PERIOD

The PWM period is specified by the PR2 register of

Timer2. The PWM period can be calculated using the

formula of Equation 11-1.

EQUATION 11-1:

PWM PERIOD

When TMR2 is equal to PR2, the following three events

occur on the next increment cycle:

• TMR2 is cleared

• The CCP1 pin is set. (Exception: If the PWM duty

cycle = 0%, the pin will not be set.)

• The PWM duty cycle is latched from CCPR1L into

CCPR1H.

11.3.2

PWM DUTY CYCLE

The PWM duty cycle is specified by writing a 10-bit

value to multiple registers: CCPR1L register and

CCP1<1:0> bits of the CCP1CON register. The

CCPR1L contains the eight MSbs and the CCP1<1:0>

bits of the CCP1CON register contain the two LSbs.

CCPR1L and CCP1<1:0> bits of the CCP1CON

value is not latched into CCPR1H until after the period

completes (i.e., a match between PR2 and TMR2

registers occurs). While using the PWM, the CCPR1H

Equation 11-2 is used to calculate the PWM pulse

width.

Equation 11-3 is used to calculate the PWM duty cycle

ratio.

EQUATION 11-2:

PULSE WIDTH

EQUATION 11-3:

DUTY CYCLE RATIO

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.

The 8-bit timer TMR2 register is concatenated with

either the 2-bit internal system clock (FOSC), or 2 bits of

the prescaler, to create the 10-bit time base. The system

clock is used if the Timer2 prescaler is set to 1:1.

When the 10-bit time base matches the CCPR1H and

2-bit latch, then the CCP1 pin is cleared (see

Figure 11-3).

11.3.3

PWM RESOLUTION

The resolution determines the number of available duty

cycles for a given period. For example, a 10-bit resolution

will result in 1024 discrete duty cycles, whereas an 8-bit

resolution will result in 256 discrete duty cycles.

The maximum PWM resolution is 10 bits when PR2 is

255. The resolution is a function of the PR2 register

value as shown by Equation 11-4.

EQUATION 11-4:

PWM RESOLUTION

TABLE 11-2:

EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (FOSC = 20 MHz)

TABLE 11-3:

EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (FOSC = 8 MHz)

Note:

The Timer2 postscaler (see Section 7.1

“Timer2 Operation”) is not used in the

determination of the PWM frequency.

PWM Period

PR2

() 1

[] 4TOSC •

•

(TMR2 Prescale Value)

Note:

If the pulse width value is greater than the

period the assigned PWM pin(s) will

remain unchanged.

Pulse Width

CCPR1L:CCP1CON<5:4>

()

•

TOSC

• (TMR2 Prescale Value)

Duty Cycle Ratio

CCPR1L:CCP1CON<5:4>

()

4PR2

()

-----------------------------------------------------------------------

Resolution

4PR2

()

[]

log

()

log

------------------------------------------ bits

PWM Frequency

1.22 kHz

4.88 kHz

19.53 kHz

78.12 kHz

156.3 kHz

208.3 kHz

Timer Prescale (1, 4, 16)

PR2 Value

0xFF

0x3F

0x1F

0x17

Maximum Resolution (bits)

6.6

PWM Frequency

1.22 kHz

4.90 kHz

19.61 kHz

76.92 kHz

153.85 kHz

200.0 kHz

Timer Prescale (1, 4, 16)

PR2 Value

0x65

0x19

0x0C

0x09

Maximum Resolution (bits)

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