PIRA32 Microcontroller
© 2016 Pira.cz
m32.pdf - page 7
3
DESIGN NOTES
3.1
Device reset
A reset is generated by holding the /RESET pin low.
The device has a noise filter in the reset path which
detects and ignores small pulses.
A reset pulse is generated on-chip whenever V
DD
rises above a certain threshold. This allows the
device to start in the initialized state when V
DD
is
adequate for operation.
To take advantage of this feature, tie the /RESET
pin through a resistor (1k to 10k) to V
DD
. This will
eliminate external RC components usually needed
to create a reset delay.
Figure 4.1
– Device reset.
When the device starts normal operation (i.e., exits
the reset condition), device operating parameters
(voltage, temperature, etc.) must be met to ensure
operation. If these conditions are not met, the
device must be held in reset until the operating
conditions are met.
3.2
Crystal oscillator
The oscillator circuit should be placed on the same
side of the board as the device. The load capacitors
should be placed next to the oscillator itself.
The load capacitor value depends on the crystal
characteristics. Optimal value ensures the F
OSC
to
lie in the tolerance range given in section Electrical
characteristics. A good starting value is 22 pF for
the load capacitors.
Figure 4.2
– Crystal oscillator.
Recommended crystal type is
Auris
Q- 4,332000M-HC49US-F-30-30-D-16
or equivalent.
3.3
Power supply
The power supply must be bypassed close to the
device with a 100 nF ceramic capacitor. The output
RDS level varies proportionally to the supply
voltage.
3.4
Internal PLL
The PIRA32 Microcontroller includes an internal
phase locked loop, which synchronises the RDS
subcarrier with 19 kHz pilot tone in case of stereo
broadcast. Parameters of the PLL are controlled by
software.
Pilot tone is tied to the PILOT input pin and must
comply with TTL levels.
The PLL is active if the /SYNC pin is driven low.
This configuration makes easy to connect commonly
available clock recovery circuits (LM567) if the pilot
tone needs to be filtered from MPX signal.
The PLL should be disabled in case of mono
broadcast by driving the /SYNC pin high or by the
command
EXTSYNC
.
3.5
External TA switch
The external TA switch can set the Traffic
Announcement flag to 1. The TA flag is set to 1 if
the TA input is driven low. This can be done using
simple mechanical switch or any logic circuit.
Where the external TA switch feature is not
required, the TA pin must be tied to V
DD
through a
resistor.
3.6
RS-232 interface
The RS-232 interface is used for the device
configuration and data transfers. The RXD and TXD
pin levels are compatible with TTL. For connection
to external equipment an inverter and level
converter is required (for example MAX232).
The serial data format is given in the RDS Encoder
Technical manual (available online).
