Texas Instruments 3138 155 232931 - Manual (Page 27)

CC2420
SWRS041B Page 27 of 89
13 4-wire Serial Configuration and Data Interface
CC2420 is configured via a simple 4-wire
SPI-compatible interface (pins SI, SO,
SCLK
and CSn) where CC2420 is the slave.
This interface is also used to read and
write buffered data (see page 39). All
address and data transfer on the SPI
interface is done most significant bit first.
13.1 Pin configuration
The digital inputs SCLK, SI and CSn are
high-impedance inputs (no internal pull-
up) and should have external pull-ups if
not driven. SO is high-impedance when
CSn
is high. An external pull-up should be
used at SO to prevent floating input at
microcontroller. Unused I/O pins on the
MCU can be set to outputs with a fixed ‘0’
level to avoid leakage currents.
13.2 Register access
There are 33 16-bit configuration and
status registers, 15 command strobe
registers, and two 8-bit registers to access
the separate transmit and receive FIFOs.
Each of the 50 registers is addressed by a
6-bit address. The RAM/Register bit (bit 7)
must be cleared for register access. The
Read/Write bit (bit 6) selects a read or a
write operation and makes up the 8-bit
address field together with the 6-bit
address.
In each register read or write cycle, 24 bits
are sent on the SI-line. The CSn pin (Chip
Select, active low) must be kept low during
this transfer. The bit to be sent first is the
RAM/Register bit (set to 0 for register
access), followed by the R/W bit (0 for
write, 1 for read). The following 6 bits are
the address-bits (A5:0). A5 is the most
significant bit of the address and is sent
first. The 16 data-bits are then transferred
(D15:0), also MSB first. See Figure 9 for
an illustration.
The configuration registers can also be
read by the microcontroller via the same
configuration interface. The R/W bit must
be set high to initiate the data read-back.
CC2420 then returns the data from the
addressed register on the 16 clock cycles
following the register address. The SO pin
is used as the data output and must be
configured as an input by the
microcontroller.
The timing for the programming is also
shown in Figure 9 with reference to Table
4. The clocking of the data on SI into the
CC2420 is done on the positive edge of
SCLK
. When the last bit, D0, of the 16
data-bits has been written, the data word
is loaded in the internal configuration
register.
Multiple registers may be written without
releasing CSn, as described in the Multiple
SPI access section on page 31.
The register data will be retained during
power down mode, but not when the
power-supply is turned off (e.g. by
disabling the voltage regulator using the
VREG_EN
pin). The registers can be
programmed in any order.