As indicated in paper CMOS oscillators to satisfy 802.15.4 and Bluetooth LE PHY specifications without a crystal reference
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8666473&casa_token=XDzQMVFMUyUAAAAA:TyaRw6JsGZL7fmp25j6ERjR2694NGhgHqp1bvj8CJQ0YxHSGLXt5Q54UgZGRyYoSSddrozizVA&tag=1
specifically Fig.3 (HFCLK OSC) vs Fig.6 (32kHz Timer OSC),
The 32k Timer OSC shows a better performance in term of drift comparing to 20MHz RC OSC.

RFTimer source configuration is determined by ASC bits 1151-1154:

(ASC_crossbar[1154:1151]):

• 0b0000(LF_CLOCK),

• 0b0001(HF_CLOCK),

• 0b0010(RC_2MHz),

• 0b0011(TIMER32k),

• 0b0100(LF_ext_PAD),

• 0b0101(LF_ext_GPIO), // external_clk_in_1

• 0b0110(ADC_CLK),

• 0b0111(LC_div_N),

• 0b1000(LC_2MHz),

• 0b1001(LC_1MHz_stat),

• 0b1010(LC_1MHz_dyn),

To configure RFTimer driven by 32k timer, the following changes are required:

ASC[1154-1151] = 0b0011

ASC[36] = 1; # divider_RFTimer_PT pass through

Translate to code:

set_asc_bit(1151);

set_asc_bit(1152);

set_asc_bit(36);