RFM42/43
3. Controller Interface
3.1. Serial Peripheral Interface (SPI)
The RFM42/43 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 1. 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 RFM42/43 every eight clock cycles. The timing parameters for the SPI interface are
shown in Table 9. The SCLK rate is flexible with a maximum rate of 10 MHz.
To read back data from the RFM42/43, 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 2. 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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