DRA722_17 Datasheet, PDF (375/408 Pages) Texas Instruments – Infotainment Applications Processor

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DRA722, DRA724, DRA725, DRA726
SPRS956B – MARCH 2016 – REVISED JANUARY 2017
8.6 Clock Routing Guidelines
8.6.1 32-kHz Oscillator Routing
When designing the printed-circuit board:
• Keep the crystal as close as possible to the crystal pins X1 and X2.
• Keep the trace lengths short and small to reduce capacitor loading and prevent unwanted noise
pickup.
• Place a guard ring around the crystal and tie the ring to ground to help isolate the crystal from
unwanted noise pickup.
• Keep all signals out from beneath the crystal and the X1 and X2 pins to prevent noise coupling.
• Finally, an additional local ground plane on an adjacent PCB layer can be added under the crystal to
shield it from unwanted pickup from traces on other layers of the board. This plane must be isolated
from the regular PCB ground plane and tied to the GND pin of the RTC. The plane must not be any
larger than the perimeter of the guard ring. Make sure that this ground plane does not contribute to
significant capacitance (a few pF) between the signal line and ground on the connections that run from
X1 and X2 to the crystal.
IC
X
Via to GND
1
X
2
Local ground plane
SPRS906_PCB_CLK_OSC_01
Figure 8-37. Slow Clock PCB Requirements
8.6.2 Oscillator Ground Connection
Although the impedance of a ground plane is low it is, of course, not zero. Therefore, any noise current in
the ground plane causes a voltage drop in the ground. Figure 8-38 shows the grounding scheme for slow
(low frequency) clock generated from the internal oscillator.
Copyright © 2016–2017, Texas Instruments Incorporated
Applications, Implementation, and Layout 375
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