AN3060

Layout considerations

Doc ID 16253 Rev 2

5/23

1

Layout considerations

1.1

What are some basic layout considerations for the RTC

crystal?

Layout considerations are straightforward for the 32 KHz crystals used with ST's M41 series

real-time clocks.

Figure 1.

Layout considerations

The primary consideration is lead length. The crystal should be mounted as near as

possible to the RTC to keep the traces short. A guard ring around these connections,

connected to ground, is very important to avoid picking up unwanted noise, which might

affect timekeeping.

Users should avoid running signals directly underneath this area unless a ground plane

resides between the signals and the crystal connections.

When possible, users can add an island ground plane under the area of the ring and crystal

body. This plane is on a separate layer from the ring, and isolated from the PCB ground

plane. It connects to ground at the GND pin of the RTC.

No other components are shown in the layout because the crystal is all that is required for

the RTC oscillator to work. No external load capacitors or bias resistors should be added.

CRYSTAL

XI

XO

GND

RTC

NO OTHER COMPONENTS

REQUIRED IN OSCILLATOR

CIRCUIT

MOUNT CRYSTAL AS CLOSE

TO RTC AS POSSIBLE

ADD GUARD RING AROUND

CRYSTAL CONNECTIONS,

CONNECT TO GND PIN

KEEP TRACES SHORT

IF POSSIBLE, ADD LOCAL GROUND PLANE,

ON SEPARATE LAYER, ISOLATED FROM PCB

GROUND PLANE, AND CONNECTED TO

RTC GND PIN

AM04719v1