ST52T400 datasheet(58/94 Pages) STMICROELECTRONICS

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ST52T400/T440/E440/T441

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The 16-bit Prescaler divides the master clock,

CLKM or the external signal TCLK. The Prescaler

output can be selected setting the PRESC bit of

REG_CONF6.

NOTE:

The external clock signal, applied on

TCLK pin, must have a frequency that is at least

two times smaller than the internal master clock.

When the Counter reaches the Peripheral Regis-

ter PWM_COUNT value (Compare Value), the

TIMEROUT signal changes from high to low level,

up to the next counter start.

The period of the PWM signal is obtained by using

the following formula:

T= (255-PWM_RELOAD)*TMRCLK

where TMRCLK is the output of the 16-bit pres-

caler. The duty cycle of the PWM signal is

controlled by the Output Register PWM_COUNT:

Ton =(PWM_COUNT- PWM_RELOAD)* TMRCLK

If the Output Register PWM_COUNT value is 255

the TIMEROUT signal is always at high level.

If the Output Register PWM_COUNT is 0, or less

than the PWM_RELOAD value, TIMEROUT sig-

nal is always at low level.

NOTE.

If the PWM_RELOAD value increases, the

duty cycle resolution decreases.

By using a 20 MHz Master Clock a PWM fre-

quency in the range between 1.2 Hz to 78.43 Khz

may be obtained.

NOTE:

The Timer, before using a new value of the

counter or of the reload, has to complete the previ-

ous counting. If the counter/reload value is

changed during counting, the new value of the

timer counter is used only at the end of the previ-

ous counting phase. This happens both in Timer

andinPWM mode.

WARNING:

loading new values of the reload in

the PWM_x_RELOAD registers, the PWM/Timer is

immediately set on-fly. This can cause some side

effects during the current counting cycle. The next

cycles work normally. This occurs both in Timer

andinPWM mode.

When the Timer is in Reset, or when the device is

reset, the TOUT pin goes to threestate: it is rec-

ommended to use a pull-up or a pull-down resistor

if this output is used to drive an external device.

Figure 9.4 PWM Mode with Auto Reload

255

compare

value

reload

PWM

Output

Ton

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