RC56D, RC336D, and RC144D Modem Device Sets Designer’s Guide
1154
Conexant
5-3
5. Keep analog signal (e.g., TXA1, TXA2, RIN, TELIN, TELOUT, MICM, MICV, and SPK) traces at least 10 mil thick
(preferably 12 - 15 mil).
6. Keep all other signal traces as wide as possible, at least 5 mil (preferably 10 mil). Route the signals between components
by the shortest possible path (the components should have been previously placed to allow this).
7. Route the traces between bypass capacitors to IC pins, at least 25 mil wide; avoid vias if possible.
8. Gather signals that pass between sections (typically low speed control and status signals) together and route them
between sections through a path in the isolation ground traces at one (preferred) or two points only. If the path is made
on one side only, then the isolation trace can be kept contiguous by briefly passing it to the other side of the PCB to jump
over the signal traces.
9. Avoid right angle (90 degree) turns on high frequency traces. Use smoothed radiuses or 45 degree corners.
10. Minimize the number of through-hole connections (feedthroughs/vias) on traces carrying high frequency signals.
11. Keep all signal traces away from crystal circuits.
12. Distribute high frequency signals continuously on a single trace rather than several traces radiating from one point.
13. Provide adequate clearance (e.g., 60 mil minimum) around feedthroughs in any internal planes in the DAA circuit.
14. Eliminate ground loops, which are unexpected current return paths to the power source.
5.1.4 Board Power Traces
1. For 2-layer boards, route a 200-mil wide power trace on two edges of the same side of the PCB around the border of the
circuits using the power. (Note that a digital ground trace should likewise be routed on the other side of the board.)
2. Generally, route all power traces before signal traces.
5.1.5 MCU Power
1. Identify power for MCU circuits (VDD) supply connections.
2. Place a 10 µF electrolytic or tantalum capacitor in parallel with a ceramic 0.1 µF capacitor b5V power and
ground at one or more points in the digital section. Place one set nearest to where power enters the PCB (edge
connector or power connector) and place another set at the furthest distance from where power enters the PCB. These
capacitors help to supply current surge demands by the digital circuits and prevent those surges from generating noise
on the power lines that may affect other circuits.
3. Tie MCU GND pins to digital ground.
5.1.6 MDP Power and Ground
1. Identify power for MDP digital circuits (VDD and AVDD) supply connections req3.3V.
2. Connect MDP VDD, AVDD, and other pins req3.3V digital power to the MCU +3.3V output pin (VD3.3).
3. Identify power for MDP analog circuits (AVAA) supply connection req5V.
4. Place a 10 µF electrolytic or tantalum capacitor in parallel with a 0.022µF capacitor between AVAA pin and analog
ground (or digital ground in a single ground plane design) at one or more points in the analog (or digital) section. Place
one set nearest to where power enters the PCB (edge connector or power connector) and place another set at the
furthest distance from where power enters the PCB. These capacitors help to supply current surge demands by the
circuits and prevent those surges from generating noise on the power lines that may affect other circuits.
5. Route the connection from the board +5V supply to the AVAA pin though a 10 µF inductor and to digital ground through a
0.01 µF ceramic capacitor.
6. Tie MDP GND and AGND pins to digital ground.
7. Tie MDP GND and AGND pins directly together or through a ferrite bead, which ever achieves the lowest noise floor.
5.1.7 Ground Planes
1. In a 2-layer design, provide digital and analog ground plane areas in all unused space around and under digital and
analog circuit components (exclusive of the DAA), respective, on both sides of the board, and connect them such a
manner as to avoid small islands. Connect each ground plane area to like ground plane areas on the same side at
several points and to like ground plane areas on the opposite side through the board at several points. Connect all
Summary of Contents for RC144D
Page 87: ...INSIDE BACK COVER NOTES ...
