CC2550-RTY1 datasheet(26/55 Pages) TI

26 / 55 page

CC2550

PRELIMINARY Data Sheet (Rev.1.2)

SWRS039A

Page 26 of 54

Figure 17 shows the number of bytes in the TX

FIFO when the threshold flag toggles, in the

case of FIFO_THR=13. Figure 16 shows the

flag as the FIFO is filled above the threshold,

and then drained below.

NUM_TXBYTES

GDO

Figure 16: FIFO_THR=13 vs. number of bytes

in FIFO (GDOx_CFG=0x02)

FIFO_THR

Bytes in TX FIFO

0 (0000)

1 (0001)

2 (0010)

3 (0011)

4 (0100)

5 (0101)

6 (0110)

7 (0111)

8 (1000)

9 (1001)

10 (1010)

11 (1011)

12 (1100)

13 (1101)

14 (1110)

15 (1111)

Table 19: FIFO_THR settings and the

corresponding FIFO thresholds

8 bytes

Underflow

margin

FIFO_THR=13

TXFIFO

Figure 17: Example of FIFO at threshold

Frequency Programming

The frequency programming in

CC2550 is

designed

minimize

the

programming

needed in a channel-oriented system.

To set up a system with channel numbers, the

desired channel spacing is programmed with

the

MDMCFG0.CHANSPC_M

and

MDMCFG1.CHANSPC_E

registers. The channel

spacing registers are mantissa and exponent

respectively.

The base or start frequency is set by the 24 bit

frequency word located in the FREQ2, FREQ1

and FREQ0 registers. This word will typically

be set to the centre of the lowest channel

frequency that is to be used.

The desired channel number is programmed

with the 8-bit channel number register,

CHANNR.CHAN

, which is multiplied by the

channel offset. The resultant carrier frequency

is given by:

()

(

)

()

256

−

⋅

CHANSPC

XOSC

carrier

CHANSPC

CHAN

FREQ

With a 26 MHz crystal the maximum channel

spacing is 405 kHz. To get e.g. 1 MHz channel

spacing one solution is to use 333 kHz

channel spacing and select each third channel

in CHANNR.CHAN.

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