MC68HC05E0 Datasheet, PDF (74/96 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor (HCMOS) microcontroller unit

BD1, BD0 — Baud Rate Select Bits (see Table 8-2).
PS1, PS0 — Port Select Bits
These bits should be 01 (binary) to select an I2C-bus function (see Table 8-1).
The I2C-bus Interface is controlled via the SI S Register ($0011).
Serial interface S register
Address bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
$0011
BB
ACK
TFF/
DOIT
IRQ/
ACK
NMA
SP
ST R/WB 0000 0000
BB — Bus Busy Bit
BB is a read-only bit which indicates if the I2C-bus is busy or idle. BB is set if a START condition
is seen on the data line. This implies that another I2C-bus master is using the bus, and that if the
MC68HC05E0 tries to start an I2C-bus sequence a data/clock collision will occur. BB is cleared
when the corresponding STOP condition, implying that the bus is again idle, is detected on the
bus. This flag generates no interrupt.
8
1 (set) – Busy
0 (clear) – Idle
ACK — Acknowledge Bit
ACK is a read-only bit which goes high if an acknowledge signal is given by a peripheral (via the
9th clock bit) after the MC68HC05E0 has transmitted a byte of data to it. ACK is updated at the
end of each receive sequence.
1 (set) – Acknowledgement received from peripheral
0 (clear) – Acknowledgement not received from peripheral
TFF/DOIT — Transmission Failure Flag/“Do it” Bit
Writing this bit high starts an I2C-bus transaction. Reading this bit at the end of an I2C-bus
transmit sequence shows the state of the TFF flag. If high, this indicates a mismatch between data
in the SI Data Register and the data sent on the serial data line. Writing a zero to this bit clears
and resets the Transmission Failure Flag circuitry.
Read
Write
1 (set) – Transmission error
Start Send or Receive
0 (clear) – No transmission error
Clear and reset TFF
MOTOROLA
8-8
SERIAL INTERFACE
MC68HC05E0