PIRA32 Microcontroller
© 2016 Pira.cz
m32.pdf - page 9
3.8
Output low-pass filter
The output RDS signal modulated at 57 kHz subcarrier requires no special filtering. Spurious products are kept
below -70 dB broadcast limit and the D/A conversion residues around the sampling frequency can be cut-off using
any simple low-pass filter. This may be based either on active filter or a simple LC element. For FM broadcast
purposes the low-pass filter rejection should be at least 20 dBc on the sampling frequency. It is recommended for
high quality FM broadcasting that the output filter characteristics interpolate at least these values:
15 kHz
57 kHz
360 kHz
-20 dB
0 dB
-30 dB
3.9
Expansion I
2
C bus
The PIRA32 Microcontroller supports several external devices that extend its functions and features. All of these
devices are optional, i.e. the final design is highly customizable. The devices are recognized automatically on
power-up or device reset.
The I
2
C devices supported are listed in following table:
Device name
Manufacturer
Description
24LC256
Microchip
Serial EEPROM. Non-volatile memory for storing RDS and
configuration data. Recommended for proper function.
PCF8563
NXP
Battery powered backup real-time clock (RTC).
MCP23008
Microchip
Parallel port for 16
×2 LCD display based on HD44780 driver.
MCP4551-103
Microchip
Digital potentiometer for software control of RDS output level.
3.10
Operation without EEPROM on I
2
C bus
Operation without EEPROM is possible. If EEPROM is not detected on the I
2
C bus after power-up or reset, the
RAM memory is filled with blank data. The RDS generator is turned off. The user may fill the RAM with valid
values using RS-232 ASCII commands and then turn on the RDS generator by the command
RDSGEN=1
.
