AN2262 Motorola / Freescale Semiconductor, AN2262 Datasheet - Page 18

• Manufacturer Part Number: AN2262
• Description: Wireless HC08 Modem
• Manufacturer: Motorola / Freescale Semiconductor

AN2262/D
AGC Capacitor
Discharge
SPI Interface
RF Protocol
18
A new phenomena is introduced when MC33493 and MC33591/3 chips are
assembled into one application. MC33591/3 (as an ASK (OOK) system) the
demodulation is performed by an envelope detector. This block (and also some
others along the signal path) are sensitive to the signal strength and they also
employ a certain time constant. In the other words, if the signal path is
saturated by a strong (i.e., local) signal, it takes a certain amount of time to
establish a full sensitivity to weak (distant) signals.
To help this situation and avoid ‘deafness’ of MC33591/3 (e.g., immediately
after data is sent out of the local module) a discharge circuit is designed. The
C13 capacitor becomes charged depending on the signal strength. If the input
signal ceases, the voltage on C13 gradually decreases.
An additional resistor, R15, is connected to an I/O pin of the microprocessor.
Normally this I/O pin stays in input mode (high impedance, Hi-Z). After data is
sent out and re-establishing the full receiver sensibility is required, the I/O pin
is switched into output mode and the output level is set to ‘0’ (low). This
effectively discharges the C13 capacitor. Then, the I/O pin function is reverted
back to input mode.
For configuration and received data transfer, a standard SPI is used. The
receiver’s SPI lines (MOSI, MISO, and SPCLK) are connected to their
respective pins of the CPU SPI module. The RESETB line, which determines
between configuration and reception modes, is connected to a regular I/O pin
of the CPU. For more details, refer to
MC33591/3 Data Sheet (Motorola order number, MC33591/D).
The RF-08 application uses a very simple protocol on the RF medium. The use
of the MC33591/3 receiver implies several fixed attributes of the RF protocol:
Since the ‘strobing’ feature is not used on the receiving side (STROBE pin is
tied high, forcing the MC33591/3 to receive at all times), no provision for waking
up needs to be done. Refer to the MC33591/3 Data Sheet (Motorola order
number, MC33591/D) for details)
Assuming all these conditions have been met, the frames conforming to the
protocol shown in
•
•
•
•
•
9600 baud maximum baud rate, ASK (OOK) modulation
Manchester encoding
An AGC settling field must appear prior to any reception
An ID (identification) field must appear right before the data field
An EOM (end-of-message) field must appear right after the data field
Wireless HC08 Modem
Figure 9
are transferred.
SPI Communication
and to the
MOTOROLA