[FAQ（常见问题解答）] FAQ_ BlueNRG-X如何测试低速时钟的频偏

本文作者：ST工程师Joshua Zhu

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FAQ_ BlueNRG-X如何测试低速时钟的频偏 .pdf (371.15 KB)
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关键词：BlueNRG-1/-2, low crystal 

问题：客户经常反馈，板子可以正常工作，但是无法测量到32khz的时钟。

ST工程师解答: 这里我展示了32khz 时钟的具体测试方法： 

1. 测试过程： 

1) Connect an oscilloscope probe in the pin 14 (ANATEST1) test point. 

2) Power up the BlueNRG-1 platform. Set the scope to capture a consistent number of 32 kHz waveform periods (for example 64 cycles, so set the time base at 200 μs). In this way, the influence of the jitter in the measure is minimized. 

3) In the IFR tool of the GUI, perform a “Read” of the current IFR configuration, then tick the check-box “LS crystal measure” and then perform a “Write” operation. Now a power cycle is required to let the new IFR be operative. 

(note: the firmware the device programmed is DTM_UART.bin or DTM_SPI.bin ,which is located in C:\Program Files (x86)\STMicroelectronics\BlueNRG-1_2 DK 3.1.0\Firmware\BLE_Examples\DTM\BlueNRG-1). 

4) At this point, the 32.768 kHz waveform is visible on the oscilloscope screen. Perform the measurement of the frequency: the difference between the target value (f = 32.768 kHz) and the measured one is the frequency offset Δf. 

2. 期望结果： 

If DUT frequency > 32.768 kHz → increases XTAL caps 

If DUT frequency < 32.768 kHz → decreases XTAL caps 

To find the oscillator ppm, use the following formula: 

where: 

Δf = offset 

f = 32.768 kHz 

Add to the found ppm value the one declared in the adopted crystal datasheet. 

当BLE堆栈初始化时，这个最终值必须在从SCA和主SCA字段中使用。