RFM24W Datasheet, PDF (13/34 Pages) List of Unclassifed Manufacturers – ISM Transceiver module

RFM24W
3. Controller Interface
3.1. Serial Peripheral Interface (SPI)
The RFM24W communicates with the host MCU over a standard 4-wire serial peripheral interface (SPI): SCLK,
SDI, SDO, and nSEL. The SPI interface is designed to operate at a maximum of 10 MHz. The SPI timing
parameters are demonstrated in Table 8. The host MCU writes data over the SDI pin and can read data from the
device on the SDO output pin. Figure 3 demonstrates an SPI write command. The nSEL pin should go low to
initiate the SPI command. The first byte of SDI data will be one of the firmware commands followed by n bytes of
parameter data which will be variable depending on the specific command. The rising edges of SCLK should be
aligned with the center of the SDI data.
Table 8. Serial Interface Timing Parameters
Symbol
Parameter
Min (ns)
Diagram
t_CH
Clock high time
40
t_CL
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
nSEL
SDO
SDI
FW Command
Param Byte 0
Param Byte n
SCLK
Figure 3. SPI Write Command
The RFM24W contains an internal MCU which controls all the internal functions of the radio. For SPI read
commands a typical MCU flow of checking clear-to-send (CTS) is used to make sure the internal MCU has
executed the command and prepared the data to be output over the SDO pin. Figure 4 demonstrates the general
flow of an SPI read command. Once the CTS value reads FFh then the read data is ready to be clocked out to the
host MCU. The typical time for a valid FFh CTS reading is 20 µs. Figure 5 demonstrates the remaining read cycle
after CTS is set to FFh. The internal MCU will clock out the SDO data on the negative edge so the host MCU
should process the SDO data on the rising edge of SCLK.
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