C8051F120 Datasheet, PDF (282/356 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
20.5. Serial Clock Timing
Four combinations of serial clock phase and polarity can be selected using the clock control bits in the
SPI0 Configuration Register (SPI0CFG). The CKPHA bit (SPI0CFG.5) selects one of two clock phases
(edge used to latch the data). The CKPOL bit (SPI0CFG.4) selects between an active-high or active-low
clock. Both master and slave devices must be configured to use the same clock phase and polarity. SPI0
should be disabled (by clearing the SPIEN bit, SPI0CN.0) when changing the clock phase or polarity. The
clock and data line relationships for master mode are shown in Figure 20.5. For slave mode, the clock and
data relationships are shown in Figure 20.6 and Figure 20.7. Note that CKPHA must be set to ‘0’ on both
the master and slave SPI when communicating between two of the following devices: C8051F04x,
C8051F06x, C8051F12x, C8051F31x, C8051F32x, and C8051F33x
The SPI0 Clock Rate Register (SPI0CKR) as shown in Figure 20.10 controls the master mode serial clock
frequency. This register is ignored when operating in slave mode. When the SPI is configured as a master,
the maximum data transfer rate (bits/sec) is one-half the system clock frequency or 12.5 MHz, whichever is
slower. When the SPI is configured as a slave, the maximum data transfer rate (bits/sec) for full-duplex
operation is 1/10 the system clock frequency, provided that the master issues SCK, NSS (in 4-wire slave
mode), and the serial input data synchronously with the slave’s system clock. If the master issues SCK,
NSS, and the serial input data asynchronously, the maximum data transfer rate (bits/sec) must be less
than 1/10 the system clock frequency. In the special case where the master only wants to transmit data to
the slave and does not need to receive data from the slave (i.e. half-duplex operation), the SPI slave can
receive data at a maximum data transfer rate (bits/sec) of 1/4 the system clock frequency. This is provided
that the master issues SCK, NSS, and the serial input data synchronously with the slave’s system clock.
SCK
(CKPOL=0, CKPHA=0)
SCK
(CKPOL=0, CKPHA=1)
SCK
(CKPOL=1, CKPHA=0)
SCK
(CKPOL=1, CKPHA=1)
MISO/MOSI
NSS (Must Remain High
in Multi-Master Mode)
MSB
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Figure 20.5. Master Mode Data/Clock Timing
282
Rev. 1.3