CLKM Frequency Change

This section describes, how the frequency of the CLKM signal is controlled. The CLKM signal is available at pin TRX_PIN_CLKM in all states except SLEEP, RX_ON_NOCLK, RX_AACK_ON_NOCLK, BUSY_RX_AACK_NOCLK. The frequency of the CLKM signal can be modified in two ways:

The sequence PHY_CHANGE_CLKM_FAST can be used, if a clock change can happen on-the-fly, e.g. if a GPIO or a timer input is driven.
If the CLKM signal is used as clock source for the MCU (clkrate={1,...,5}) and therefore no on the fly frequency change is allowed, the sequence PHY_CHANGE_CLKM_SAFE is used doing a clock change using a wake-sleep-wake transition.

Parameters:

clkrate

This parameter represents the frequency value of the CLKM signal, which is generated at pin TRX_PIN_CLKM (see table below).

CLKM Frequencies

clkrate	f_CLKM
0	OFF, CLKM = LOW.
1	1 MHz
2	2 MHz
3	4 MHz
4	8 MHz
5	16 MHz
6	1/4 MHz=250 kHz
7	IEEE 802.15.4 symbol rate BPSK_20 : 20 kHz BPSK_40 : 40 kHz O-QPSK_{100,200,400} : 25 kHz O-QPSK_{250,500,1000} : 62.5 kHz

Use Cases:

PHY_CHANGE_CLKM_FAST

With this sequence, the clock signal at TRX_PIN_CLKM is changed immediately.

Code example

    /* AT86RF212::[ACTIVE] */
    trx_bit_write(SR_CLKM_SHA_SEL, 0);
    trx_bit_write(SR_CLKM_CTRL, clkrate);

PHY_CHANGE_CLKM_SAFE

With this sequence, the clock signal at TRX_PIN_CLKM changes after awake, if the value of the parameter clkrate is not {0,6,7}.

Note:: The prep_clkchange() function is MCU dependent and is responsible to bring the MCU to a state, where a clock change is allowed. After asserting TRX_PIN_SLP_TR = HIGH, the CLKM signal delivers 35 (refer to tTR3) cycles before it is turned off (CLKM = LOW, see also PHY_STATE_TRX_OFF__SLEEP).

Code example

    /* AT86RF212::TRX_OFF */
    trx_bit_write(SR_CLKM_SHA_SEL, 1);
    trx_bit_write(SR_CLKM_CTRL, clkrate);
    trx_pinset_slptr(1);
    /* AT86RF212::SLEEP */
    prep_clkchange();
    trx_pinset_slptr(0);
    delay(tTR2);
    /* AT86RF212::TRX_OFF */