MC68HC05E0 Datasheet, PDF (68/96 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor (HCMOS) microcontroller unit

The sequence of events for starting a send/receive sequence in either SPI or I2C-bus mode is as
follows:
1) Write to SI Mode register ($0010). The state of bits 0 and 1 select the data
and clock lines, and the protocol (see Table 8-1).
2) If sending data from the MCU to a peripheral, write the data to be sent to the
SI Data register ($000F).
3) Start the send/receive sequence by writing a “1” to the DOIT bit (bit 5 in the
Serial Interface Control/Status Register, the SI S Register, $0011).The SI S
Register also contains control bits to determine the data direction, and
whether STOP/START bits are to be added to the data (I2C-bus only).
The transmission rate in SPI and I2C-bus mode is determined by BD0 and BD1 (bits 2 and 3 in
the SI Mode Register) which control the division ratio applied to the subsystem clock (see
Figure 8-1). The division ratios are listed in Table 8-2. Note that the maximum SI transfer rate =
P02 ÷ 6 (i.e. when SC0, SC1, BD0 and BD1 are all set to 1).
Sub-system
8
Clock
÷3
÷2
÷2
÷2
÷2
0
1
MUX
2
3
SPI / I ^2C-Clock
(Baud Rate)
BD1 BD0
Figure 8-1 Serial Port Baud Rate Generation
Table 8-2 BD0, BD1: Serial Port Transfer Rate Selection
BD1
BD0
0
0
0
1
1
0
1
1
SI transfer rate
f_SUB ÷ 48
f_SUB ÷ 24
f_SUB ÷ 12
f_SUB ÷ 6
MOTOROLA
8-2
SERIAL INTERFACE
MC68HC05E0