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According to TI CC2545 User Guide Page 236, Section:23.5 Frequency and Channel Programming:

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For normal transmit and receive tasks, the carrier frequency is set by using the register PRF_CHAN.FREQ. The carrier frequency is 2379 + n MHz, where n is the value of this register, where n can be from 0 to 116. This gives a frequency range from 2379 MHz to 2495 MHz. Note that this frequency range extends beyond the ISM band. If PRF_CHAN.FREQ is set to 127, and for the Rx and Tx test commands, the frequency must instead be programmed directly in hardware registers. In this case, the synthesizer frequency is set by programming the 7 bit frequency word located in FREQCTRL.FREQ[6:0]. The synthesizer frequency is given by 2379+ nMHz, where n is the value of FREQCTRL.FREQ[6:0], and is programmable in 1-MHz steps. The device supports synthesizer frequencies in the range from 2379 MHz to 2495 MHz. The usable settings for freq[6:0] is consequently 0 to 116. In Rx, the system operates on a low intermediate frequency (IF) of 1 MHz for data rates up to 1 Mbps, and on a zero IF for 2 Mbps. In Tx, the system supports operating on low IF or zero IF. The IF to be used for Tx can be programmed in the register MDMTEST1.TX_TONE. The receiver may operate on a positive of negative IF when the data rate is 1 Mbps and lower; this is controlled with MDMTEST1.RX_IF. When the symbol rate is 1 Mbps or lower and the LLE programs the frequency, it uses a ±1 MHz IF on Tx. For both Rx and Tx, a negative IF is used when PRF_CHAN.FREQ < 62 and a positive IF is used when PRF_CHAN.FREQ ≥ 62 When the symbol rate is 2 Mbps and the LLE programs the frequency, it uses an IF on Tx as specified in PRF_RADIO_CONF.TXIF. This IF may be zero, or ±1 MHz, ±2 MHz, or ±3 MHz. The recommended setting is ±1 MHz. A negative IF is used when PRF_CHAN.FREQ < 62 and a positive IF is used when PRF_CHAN.FREQ ≥ 62. For all data rates, the setting of MDMCTRL1.PHASE_INVERT is taken into account by the LLE when finding the setting for MDMTEST1.TX_TONE. The FREQCTRL register will be programmed corresponding to the programmed IF in order to operate on the channel specified by PRF_CHAN.FREQ.

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My questions is:

[1] How many number of channels are possible within 2400 - 2483.5 ISM band if I use 1Mbps and/or 2 Mbps Data rates.

[2] Could I change Channel Spacing for both 1 Mbps or 2 Mbps. Do CC2545 support channel spacing settings.?

I have consulted Datasheet and User guide but i am still confused some brief description will be appreciated. Thank you

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It says right there in the data sheet text that you copied:

"...the carrier frequency is set by using the register PRF_CHAN.FREQ. The carrier frequency is 2379 + n MHz, where n is the value of this register, where n can be from 0 to 116. This gives a frequency range from 2379 MHz to 2495 MHz."

So there you have it. There are 117 channels possible. Obviously you have to check if all those channel frequencies are legal for the band in your area and at what power level.

You also have to take into account the data modulation rate. Clearly if you are using a data rate up to 2Mbps you will likely be limited to half or less number of channels. If half you would have to confine usage to carriers of the odd frequency numbers or even frequency numbers.

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