AN95089 Datasheet, PDF (9/19 Pages) Ramtron International Corporation – BLE Crystal Oscillator Selection and Tuning Techniques

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques
Figure 6. ppm Variation Across Boards for the Same CL
ppm variation across boards
4
2
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14
-2
-4
-6
-8
-10
Board#
From this data, it is evident that the variation in crystal accuracy across boards is in the range of ± 7-8 ppm. This
range of variation is permissible for optimum RF performance. Therefore, you don’t need to tune each board. You can
tune a board once and use the same capacitance trim value for boards with the same PCB layout.
After the trim value is determined for a device, the trim value should be written to the ECO trim register,
CYREG_BLE_BLERD_BB_XO_CAPTRIM, using the register write command in Step 5. The register must be written at
the device initialization stage. It is important to note that the ECO trim register is written with a default value as part of
the BLE stack initialization. Therefore, the trim value should be written to the ECO trim register after the BLE stack
initialization in the application, so that the register holds the updated trim value. This sequence is shown below.
int main()
{
/* Stack Initialization */
CyBle_Start(EventCallback);
/* Write trim value */
*(uint32*) CYREG_BLE_BLERD_BB_XO_CAPTRIM = <CAP TRIM VALUE >;
for(;;)
{
CyBle_ProcessEvents();
}
}
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Document No. 001-95089 Rev. *A
9