ST7LITE29 datasheet(56/131 Pages) STMICROELECTRONICS

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ST7LITE2

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12-BIT AUTORELOAD TIMER (Cont’d)

11.2.3 Functional Description

PWM Mode

This mode allows up to four Pulse Width Modulat-

ed signals to be generated on the PWMx output

pins. The PWMx output signals can be enabled or

disabled using the OEx bits in the PWMCR regis-

ter.

PWM Frequency and Duty Cycle

The four PWM signals have the same frequency

(fPWM) which is controlled by the counter period

and the ATR register value.

fPWM = fCOUNTER / (4096 - ATR)

Following the above formula,

– If fCOUNTER is 32 MHz, the maximum value of

fPWM is 8 MHz (ATR register value = 4092), the

minimum value is 8 KHz (ATR register value = 0)

– If fCOUNTER is 4 Mhz, the maximum value of fPWM

is 2 MHz (ATR register value = 4094),the mini-

mum value is 1 KHz (ATR register value = 0).

Note: The maximum value of ATR is 4094 be-

cause it must be lower than the DCR value which

must be 4095 in this case.

At reset, the counter starts counting from 0.

When a upcounter overflow occurs (OVF event),

the preloaded Duty cycle values are transferred to

the Duty Cycle registers and the PWMx signals

are set to a high level. When the upcounter match-

es the DCRx value the PWMx signals are set to a

low level. To obtain a signal on a PWMx pin, the

contents of the corresponding DCRx register must

be greater than the contents of the ATR register.

The polarity bits can be used to invert any of the

four output signals. The inversion is synchronized

with the counter overflow if the TRAN bit in the

TRANCR register is set (reset value). See Figure

35.

Figure 35. PWM Inversion Diagram

The maximum available resolution for the PWMx

duty cycle is:

Resolution = 1 / (4096 - ATR)

Note: To get the maximum resolution (1/4096), the

ATR register must be 0. With this maximum reso-

lution, 0% and 100% can be obtained by changing

the polarity.

Figure 36. PWM Function

PWMx

counter

overflow

OPx

PWMxCSR Register

inverter

DFF

TRAN

TRANCR Register

DUTY CYCLE

AUTO-RELOAD

4095

000

WITH OE=1

AND OPx=0

(ATR)

(DCRx)

WITH OE=1

AND OPx=1

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