SI4455 Datasheet, PDF (23/40 Pages) Silicon Laboratories – 20-Pin 3x3 mm QFN package

Si4455
The Si4455 contains an internal MCU which controls all the internal functions of the radio. For SPI read
commands, a typical communication 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 9 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 10 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.
Send Command
Read CTS
0xFF
CTS Value
0x00
Retrieve
Response
nSEL
SDO
CTS
SDI
ReadCmdBuff
SCLK
Figure 9. SPI Read Command—Check CTS Value
nSEL
SDO
SDI
SCLK
Response Byte 0
Response Byte n
Figure 10. SPI Read Command—Clock Out Read Data
Rev 1.1
23