RFM22 Datasheet, PDF (14/150 Pages) –

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
Symbol
t_CH
t_CL
Table 10. Serial Interface Timing Parameters
Parameter
Min
Diagram
Clock high time
40
Clock low time
40
t_DS
Data setup time
20
t_DH
Data hold time
20
t_DD
Output data delay time 20
t_EN
Output enable time 20
t_DE
Output disable time 50
t_SS
Select setup time
20
t_SH
Select hold time
50
t_SW
Select high period 80
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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