MCCIRM Freescale Semiconductor / Motorola, MCCIRM Datasheet - Page 48

• Manufacturer Part Number: MCCIRM
• Description: MCCIRM Multi-Channel Communications Interface Reference Manual
• Manufacturer: Freescale Semiconductor / Motorola

SPI Submodule
4.7 SPI Serial Clock Baud Rate
Reference Manual
48
For a master, writing to the SPDR initiates the transfer. For a slave, the
first edge of SCK indicates the start of a transfer. The SPI is left-shifted
on the first and each succeeding odd clock edge, and data is latched on
the second and succeeding even clock edges.
SCK is inactive for the last half of the eighth SCK cycle. For a master,
SPIF is set at the end of the eighth SCK cycle (after the 17th SCK edge).
Since the last SCK edge occurs in the middle of the eighth SCK cycle,
however, the slave has no way of knowing when the end of the last SCK
cycle occurs. The slave, therefore, considers the transfer complete after
the last bit of serial data has been sampled, which corresponds to the
middle of the eighth SCK cycle.
When CPHA is 1, the SS line may remain at its active low level between
transfers. This format is sometimes preferred in systems having a single
fixed master and only one slave that needs to drive the MISO data line.
Baud rate is selected by writing a value from 2 to 255 into the BAUD field
in the SPCR of the master MCU. (Writing a BAUD value into the SPCR
of the slave device has no effect.) The SPI uses a modulus counter to
derive SPI serial clock (SCK) baud rate from the MCU system clock.
This equation determines the SCK baud rate:
Therefore, given a desired SCK baud rate, compute BAUD like this:
Giving BAUD a value of 0 or 1 disables the baud rate generator. SCK is
disabled and assumes its inactive state value.
BAUD has 254 active values.
values and the corresponding SCK frequency based on a 16.78-MHz
system clock.
Freescale Semiconductor, Inc.
For More Information On This Product,
SCK baud rate = system clock/(2 x BAUD)
BAUD = system clock/(2 x SCK baud rate)
Go to: www.freescale.com
SPI Submodule
Multichannel Communication Interface — Rev. 1.0
Table 4-3
lists several possible baud
MOTOROLA