ST92185B STMicroelectronics, ST92185B Datasheet - Page 157

• Manufacturer Part Number: ST92185B
• Description: 16k/24k/32k Rom Hcmos Mcu With On-screen-display
• Manufacturer: STMicroelectronics

VOLTAGE SYNTHESIS (Cont’d)
PWM Generation
The counter increments continuously, clocked at
INTCLK divided by 4. Whenever the 7 least signif-
icant bits of the counter overflow, the VS output is
set.
The state of the PWM counter is continuously
compared to the value programmed in the 7 most
significant bits of the tuning word. When a match
occurs, the output is reset thus generating the
PWM output signal on the VS pin.
This Pulse Width modulated signal must be fil-
tered, using an external RC network placed as
close as possible to the associated pin. This pro-
vides an analog voltage proportional to the aver-
age charge passed to the external capacitor. Thus
for a higher mark/space ratio (High time much
Figure 99. PWM Generation
PWM OUTPUT
COUNTER
7-BIT PWM
VALUE
000
127
OVERFLOW
ST92185B - VOLTAGE SYNTHESIS TUNING CONVERTER (VS)
INTCLK/4 x 128
greater than Low time) the average output voltage
is higher. The external components of the RC net-
work should be selected for the filtering level re-
quired for control of the system variable.
OVERFLOW
Figure 98. Typical PWM Output Filter
PWM OUT
R
1K
ext
C
OVERFLOW
ext
OUTPUT
VOLTAGE
t
t
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