Introduction
The KW45-EVK & FRDM-MCX W71 include an RSIM (Radio System Integration Module) module with an external 32 MHz crystal oscillator and 32kHz external oscillator. 32MHz clock source reference is mainly intended to supply the Bluetooth LE Radio peripheral, but it can be used as the main clock source of the MCU as well. This oscillator includes a set of programmable capacitors to support crystals with different load capacitance needs. Changing the value of these capacitors can modify the frequency the oscillator provides, that way, the central frequency can be tuned to meet the wireless protocol standards. This configurable capacitance range is from C: 3.74pF to C: 10.67pF and it is configured through the RFMC Register XO_Test field at the CDAC. The KW45 comes preprogrammed with a default load capacitance value (0x1Eh). However, since there is variance in devices due to tolerances and parasite effects, the correct load capacitance should be checked by verifying that the optimal central frequency is attained. You will need a spectrum analyzer to measure the central frequency. To find the most accurate value for the load capacitance, it is recommended to use the Connectivity Test demo application. 32kHz clock source reference is mainly intended to run in low power when the 32MHz clock is switched off. This 32kHz clock enable to leave the low power mode and enter in Bluetooth LE events.
Adjusting 32MHz Frequency Example
- Program the KW45 /MCX W71 Connectivity Test software on the device. This example can be found in SDK_2_15_000_KW45B41Z-EVK_MR5\boards\kw45b41zevk\wireless_examples\genfsk\connectivity_test folder from your SDK package. Baremetal and FreeRTOS versions are available.
- In case that KW45-EVK board is being used to perform the test, you should move the 15pF capacitor populated in C3 to C4, to direct the RF signal on the SMA connector.
3. Connect the board to a serial terminal software. When you start the application, you will be greeted by the NXP logo screen:
- Press the enter key to start the test. Then press "1" to select "Continuous tests":
5. Finally, select "6" to start a continuous unmodulated RF test. At this point, you should be able to measure the signal in the spectrum analyzer. You can change the RF channel from 0 to 127 ("q" Ch+ and "w" Ch- keys), which represents the bandwidth from 2.360GHz to 2.487GHz, stepping of 1MHz between two consecutive channels. To demonstrate the trimming procedure, this document will make use of channel 42 (2.402GHz) which corresponds to the Bluetooth LE channel 37. In this case, with the default capacitance value, our oscillator is not exactly placed at the center of the 2.402GHz, instead, it is slightly deflected to 2.40200155 GHz, as depicted in the following figure:
6. The capacitance can be adjusted with the "d" XtalTrim+ and "f" XtalTrim- keys. Increasing the capacitance bank means a lower frequency. In our case, we need to increase the capacitance to decrease the frequency. The nearest frequency of 2.402 GHz was 2.40199940 GHz
7. Once the appropriate XTAL trim value has been found, it can be programmed as default in any Bluetooth LE example, changing the BOARD_32MHZ_XTAL_CDAC_VALUE constant located in the board_platform.h file:
Adjusting 32kHz Frequency Example
You could adjust the capacitor bank on the 32kHz oscillator.
You need to observe the 32kHz frequency at pin 45 (PTC7) using an spectrum analyzer or a frequency meter.
Inserting this below code in the main(void) in your application: Hello_world application in this example. 32kHz frequency is not active by default on pin45(PTC7).
You need to configure the OSC32K_RDY at 1 in the CCM32K register Status Register (STATUS) field to observe the 32kHz frequency at pin 45 (PTC7).
Configure the CAP_SEL, XTAL_CAP_SEL and EXTAL_CAP_SEL field available in the CCM32K register 32kHz Oscillator Control Register (OSC32K_CTRL).
XTAL_CAP_SEL and EXTAL_CAP_SEL values are from 0pF (0x00h) to 30pF (0x0Fh). You could configure those 2 registers in the clock_config.c file. Default values are 8pF for both registers.
