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C165UTAH
IOM-2 Interface Controller
Data Sheet
355
2001-02-23
Before starting a transmission, the
μ
CT should verify that the transmitter is inactive, i.e.
that a possible previous transmission has been terminated. This is indicated by a ’0’ in
the MONITOR Channel Active MAC status bit.
After having written the MONITOR Data Transmit (MOX) register, the
μ
CT sets the
MONITOR Transmit Control bit MXC to ’1’. This enables the MX bit to go active (0),
indicating the presence of valid MONITOR data (contents of MOX) in the corresponding
frame. As a result, the receiving device stores the MONITOR byte in its MONITOR
Receive MOR register and generates an MDR interrupt status (MRE must be ’1’).
Alerted by the MDR interrupt, the
μ
CR reads the MONITOR Receive (MOR) register.
When it is ready to accept data (e.g. based on the value in MOR, which in a point-to-
multipoint application might be the address of the destination device), it sets the MR
control bit MRC to ’1’ to enable the receiver to store succeeding MONITOR channel
bytes and acknowledge them according to the MONITOR channel protocol. In addition,
it enables other MONITOR channel interrupts by setting MONITOR Interrupt Enable
(MIE) to ’1’.
As a result, the first MONITOR byte is acknowledged by the receiving device setting the
MR bit to ’0’. This causes a MONITOR Data Acknowledge MDA interrupt status at the
transmitter.
A new MONITOR data byte can now be written by the
μ
CT in MOX. The MX bit is still in
the active (0) state. The transmitter indicates a new byte in the MONITOR channel by
returning the MX bit active after sending one frame in the inactive state. As a result, the
receiver stores the MONITOR byte in MOR and generates a new MDR interrupt status.
When the
μ
CR has read the MOR register, the receiver acknowledges the data by
returning the MR bit active after sending one frame in the inactive state. This in turn
causes the transmitter to generate an MDA interrupt status.
This "MDA interrupt – write data – MDR interrupt – read data – MDA interrupt"
handshake is repeated as long as the transmitter has data to send.
When the last byte has been acknowledged by the receiver (MDA interrupt status), the
microcontroller sets the MONITOR Transmit Control bit MXC to ’0’. This enforces an
inactive (’1’) state in the MX bit. Two frames of MX inactive signifies the end of a
message. Thus, a MONITOR Channel End of Reception MER interrupt status is
generated by the receiver when the MX bit is received in the inactive state in two
consecutive frames. As a result, the microcontroller sets the MR control bit MRC to 0,
which in turn enforces an inactive state in the MR bit. This marks the end of the
transmission, making the MONITOR Channel Active MAC bit return to ’0’.
The MONITOR transfer protocol rules are summarized in the following section
A pair of MX and MR in the inactive state for two or more consecutive frames indicates
an
idle state
or an
end of transmission
.
A
start of a transmission
is initiated by the transmitter by setting the MXC bit to ’1’
enabling the internal MX control. The receiver acknowledges the received first byte by
setting the MR control bit to ’1’ enabling the internal MR control.