ST7LITE3 datasheet(56/167 Pages) STMICROELECTRONICS

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ST7LITE3

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DUAL 12-BIT AUTORELOAD TIMER 3 (Cont’d)

11.2.3 Functional Description

11.2.3.1 PWM Mode

This mode allows up to four Pulse Width Modulat-

ed signals to be generated on the PWMx output

pins.

PWM Frequency

The four PWM signals can have the same fre-

quency (fPWM) or can have two different frequen-

cies. This is selected by the ENCNTR2 bit which

enables single timer or dual timer mode (see Fig-

ure 33 and Figure 34).

The frequency is controlled by the counter period

and the ATR register value. In dual timer mode,

PWM2 and PWM3 can be generated with a differ-

ent frequency controlled by CNTR2 and ATR2.

fPWM = fCOUNTER / (4096 - ATR)

Following the above formula,

– 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).

Duty Cycle

The duty cycle is selected by programming the

DCRx registers. These are preload registers. The

DCRx values are transferred in Active duty cycle

registers after an overflow event if the correspond-

ing transfer bit (TRANx bit) is set.

The TRAN1 bit controls the PWMx outputs driven

by counter 1 and the TRAN2 bit controls the

PWMx outputs driven by counter 2.

PWM generation and output compare are done by

comparing these active DCRx values with the

counter.

The maximum available resolution for the PWMx

duty cycle is:

Resolution = 1 / (4096 - ATR)

where ATR is equal to 0. With this maximum reso-

lution, 0% and 100% duty cycle can be obtained

by changing the polarity.

At reset, the counter starts counting from 0.

When a upcounter overflow occurs (OVF event),

the preloaded Duty cycle values are transferred to

the active Duty Cycle registers and the PWMx sig-

nals are set to a high level. When the upcounter

matches the active DCRx value the PWMx signals

are set to a low level. To obtain a signal on a

PWMx pin, the contents of the corresponding ac-

tive DCRx register must be greater than the con-

tents of the ATR register.

The maximum value of ATR is 4094 because it

must be lower than the DCR value which must be

4095 in this case.

Polarity Inversion

The polarity bits can be used to invert any of the

four output signals. The inversion is synchronized

with the counter overflow if the corresponding

transfer bit in the ATCSR2 register is set (reset

value). See Figure 35.

Figure 35. PWM Polarity Inversion

The Data Flip Flop (DFF) applies the polarity inver-

sion when triggered by the counter overflow input.

Output Control

The PWMx output signals can be enabled or disa-

bled using the OEx bits in the PWMCR register.

PWMx

counter

overflow

OPx

PWMxCSR Register

inverter

DFF

TRANx

ATCSR2 Register

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