Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

 

(1)  In practice the input capacitor C

T

  will in most cases be a fixed capacitor 

 

Alternatively to the last point listed above and in order to achieve highest noise immunity, 
a guard ring can be placed around the crystal which must be tied to ground to isolate the 
crystal from unwanted noise pickup. It should be tied to V

SS

 of the real time clock at one 

place only to avoid unforeseen currents running via the guard ring. In addition another 
local ground plane on an adjacent PCB layer can be added under the crystal. Also this 
ground plane should be isolated from the regular PCB ground plane and connected to 
V

SS

 of the RTC. The dimensions of this ground plane shouldn’t be much larger than the 

perimeter of the guard ring, but optimally should include the complete crystal and RTC. 
Be aware that this ground plane will create parasitic capacitances on the OSCI and 
OSCO pins because on the adjacent outer layer the tracks to the crystal will run. A 
sketch of such a layout is given in Fig 15. Here the use of SMD components is assumed 
which is recommended because the smaller dimensions will result in smaller loop area’s. 

Both examples use the PCF8563. The same principles apply when one of the other 
RTCs is used, with some small modifications due to differences in pinning.