mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 275

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

12.3.5 SPI Serial Clock Baud Rate
MC68HC16Y3/916Y3
USER’S MANUAL
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 seventeenth 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 one, 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 SPBR[7:0] in the SPCR of
the master MCU. Writing a SPBR[7:0] value into the SPCR of the slave device has no
effect. The SPI uses a modulus counter to derive SCK baud rate from the MCU system
clock.
The following expressions apply to SCK baud rate:
Giving SPBR[7:0] a value of zero or one disables the baud rate generator. SCK is
disabled and assumes its inactive state value.
SPBR[7:0] has 254 active values. Table 12-4 lists several possible baud values and
the corresponding SCK frequency based on a 16.78-MHz system clock.
System Clock
Frequency
16.78 MHz
MULTICHANNEL COMMUNICATION INTERFACE
Freescale Semiconductor, Inc.
For More Information On This Product,
SPBR[7:0]
Required Division
Table 12-4 SCK Frequencies
SCK Baud Rate
Ratio
168
510
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16
34
4
8
=
------------------------------------------------------------------------- -
2 SCK Baud Rate Desired
Value of SPBR Actual SCK Frequency
or
=
255
17
84
2
4
8
------------------------------------ -
2 SPBR[7:0]
f
sys
f
sys
4.19 MHz
2.10 MHz
1.05 MHz
493 kHz
100 kHz
33 kHz
MOTOROLA
12-11