CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits
Page 13 of 42
MICRF505 data sheet). The 12-bit ‘M0’ divider controls the reference oscillator
division ratio and the channel spacing. The ‘M0’ divider spans two control registers
within the MICRF505: bits 3-0 of control register 0x0D and bits 7-0 of control register
0x0E. The formula that relates channel spacing to the ‘M0’ value is:
M0
F
F
Spacing
Channel
XCO
C
=
=
For example, a value of M0=16 will divide the 8MHz reference down to the desired
500kHz channel spacing.
A dual-modulus prescaler is used to generate the carrier frequency through
manipulation of the ‘N0’ and ‘A0’ dividers. The 12-bit N0 divider spans control
registers 0x0B (bits 3-0) and 0x0C (bits 7-0), while the 6-bit A0 divider spans control
registers 0x0A (bits 5-0).
The formula that relates carrier frequency to the ‘N0’ and ‘A0’ values is:
RF Carrier Frequency = Channel Spacing x (16N + A)
For example, with a channel spacing of 500kHz, values of ‘N0’=107 and ‘A0’=14
produce a carrier frequency of 863.0MHz. (NOTE: N0 must be larger than A0 in
order for this to work as described.)
The charge pump is set to 125
µ
A by clearing bit 7 in control register 0x02. The PLL
second order loop filter uses component values that have been calculated based on
the following parameters:
•
166.6kbps data rate
•
3kHz loop bandwidth
•
60
°
phase margin
•
67MHz/V VCO gain
•
125
µ
A charge pump current
Several third-party loop filter calculator programs are available, including a program
from Micrel called “RF TestBench”. Please consult Micrel for technical assistance
with loop filter component selection.
4.3 Microcontroller
–
MSP430F1232
The Texas Instruments MSP430F1232 microcontroller was selected for this project
because of its low power consumption and rich feature set.
The MSP430F1232 is placed in its active mode during voice communications and
performs all required system management functions. If voice is absent for more than
twenty-three seconds, the boards will enter powersave mode (MSP430F1232 “Low-
power mode 0”). The master and slave boards, if paired, will wake from powersave
mode when voice is presented at the audio input to the master board.
The microcontroller operates from a 3.3V power supply and an 8MHz crystal in this
design. The 8MHz crystal sources the microcontroller’s ‘basic clock module’ during
