RFM22
3. Controller Interface
3.1. Serial Peripheral Interface (SPI)
The RFM22 communicates with the host MCU over a 3 wire SPI interface: SCLK, SDI, and nSEL. The host
MCU can also read data from internal registers on the SDO output pin. A SPI transaction is a 16-bit sequence
which consists of a Read-Write (R/W) select bit, followed by a 7-bit address field (ADDR), and an 8-bit data field
(DATA), as demonstrated in Figure 2. The 7-bit address field supports reading from or writing to one of the 128, 8-bit
control registers. The R/W select bit determines whether the SPI transaction is a write or read transaction. If R/W = 1,
it signifies a WRITE transaction, while R/W = 0 signifies a READ transaction. The contents (ADDR or DATA) are
latched into the RFM22 every eight clock cycles. The timing parameters for the SPI interface are shown in Table 10.
The SCLK rate is flexible with a maximum rate of 10 MHz.
Figure 2. SPI Timing
Table 10. Serial Interface Timing Parameters
Symbol
Parameter
Clock high time
Clock low time
Min
Diagram
t
40
CH
t
40
CL
t
Data setup time
20
DS
t
Data hold time
Output data delay time
Output enable time
20
20
20
DH
t
DD
t
EN
t
Output disable time
Select setup time
Select hold time
50
20
50
80
DE
t
SS
t
SH
t
Select high period
SW
To read back data from the RFM22, the R/W bit must be set to 0 followed by the 7-bit address of the register from
which to read. The 8 bit DATA field following the 7-bit ADDR field is ignored when R/W = 0. The next eight negative
edge transitions of the SCLK signal will clock out the contents of the selected register. The data read from the
selected register will be available on the SDO output pin. The READ function is shown in Figure 3. After the READ
function is completed the SDO pin will remain at either a logic 1 or logic 0 state depending on the last data bit
clocked out (D0). When nSEL goes high the SDO output pin will be pulled high by internal pullup.
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