RFM22B/23B
3. Controller Interface
3.1. Serial Peripheral Interface (SPI)
The RFM22B/23B communicates with the host MCU over a standard 3-wire SPI interface: SCLK, SDI, and nSEL.
The host MCU can read data from the device 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 3. The 7-bit address field is used to select one of the 128, 8-bit control registers. The
R/W select bit determines whether the SPI transaction is a read or write 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
RFM22B/23B 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.
Data
Address
MSB
LSB
RW A6 A5 A4 A3 A2 A1 A0 D7 D6 D5 D4 D3 D2 D1 D0 xx xx RW A7
SDI
SCLK
nSEL
Figure 3. SPI Timing
Table 10. Serial Interface Timing Parameters
Symbol
Parameter
Min (nsec)
Diagram
t
Clock high time
Clock low time
40
40
20
20
20
20
50
20
50
80
CH
t
CL
DS
DH
DD
SCLK
t
Data setup time
tSS
tCL tCH
tDS tDH tDD
tSH tDE
t
t
Data hold time
SDI
Output data delay time
Output enable time
Output disable time
Select setup time
Select hold time
SDO
t
t
EN
DE
tEN
tSW
nSEL
t
SS
SH
t
t
Select high period
SW
To read back data from the RFM22B/23B, 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 on the SDI pin 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 4.
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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