ST72774 datasheet(118/144 Pages) STMICROELECTRONICS

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ST72774/ST727754/ST72734

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PWM/BRM GENERATOR (Cont’d)

BRM Generation

The BRM bits allow the addition of a pulse to widen

a standard PWM pulse for specific PWM cycles.

This has the effect of “fine-tuning” the PWM Duty

cycle (without modifying the base duty cycle), thus,

with the external filtering, providing additional fine

voltage steps.

The incremental pulses (with duration of TCPU) are

added to the beginning of the original PWM pulse.

The PWM intervals which are added to are

specified in the 4-bit BRM register and are

encoded as shown in the following table. The BRM

values shown may be combined together to

provide a summation of the incremental pulse

intervals specified.

The pulse increment corresponds to the PWM

resolution.

For example,if

– Data 18h is written to the PWM register

– Data 06h (00000110b) is written to the BRM reg-

ister

– with a 8MHz internal clock (125ns resolution)

Then 3.0

µs-long pulse will be output at 8 µs

intervals,

except

for

cycles

numbered

2,4,6,10,12,14, where the pulse is broadened to

3.125

µs.

Note: If 00h is written to both PWM and BRM registers,

the generator output will remain at “0”. Conversely,

if both registers hold data 3Fh and 0Fh, respective-

ly, the output will remain at “1” for all intervals #1 to

#15, but it will return to zero at interval #0 for an

amount of time corresponding to the PWM resolu-

tion (TCPU).

An output can be set to a continuous “1” level by

clearing the PWM and BRM values and setting

POL = “1” (inverted polarity) in the PWM register.

This allows a PWM/BRM channel to be used as an

additional I/O pin if the DAC function is not

required.

Table 28. Bit BRM Added Pulse Intervals

(Interval #0 not selected).

Figure 72. BRM pulse addition (PWM > 0)

BRM 4 - Bit Data

Incremental Pulse Intervals

0000

none

0001

i = 8

0010

i = 4,12

0100

i = 2,6,10,14

1000

i = 1,3,5,7,9,11,13,15

TCPU x 64

CPU x 64 increment

m = 1

m = 0

m = 2

CPU x 64

m = 15

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