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3-2
Six SOMA channels are associated with the three full-duplex SCCs. Each
channel is permanently assigned to service the receive or transmit operation
of one of the secs, and is always available, regardless of the sec protocol
chosen.
The SOMA channels allow flexibility in managing the data flow. On a buffer-
by-buffer basis, each sec buffer descriptor determines whether data should
be transferred between the secs and external memory or between the secs
and the on-chip dual-port RAM. The SCC to external memory path bypasses
the dual-port RAM by allowing the SOMA channel to arbitrate for the M68000
bus. The SCC to dual-port RAM path saves external memory and eliminates
the need to arbitrate for the bus.
The SOMA channels are transparent to the user, implementing bus-cycle-
stealing data transfers controlled by microcode in the communication pro-
cessor (CP) main controller. Having no user-accessible registers, the channels
do not need to be configured since they are effectively controlled by the
choice of sec configuration options.
When one SOMA channel needs to transfer data to or from external memory,
it will request the M68000 bus with the internal signal SOBR, wait for SOBG,
and assert the external signal BGACK. It remains the bus master for only one
bus cycle. The six SOMA channels have priority over the IOMA controller. If
the IOMA is bus master when an SOMA channel needs to transfer over the
M68000 bus, the SOMA will steal a cycle from the IOMA while BGACK remains
continuously low. Each SOMA channel may be programmed with a separate
function code if desired. If a bus error occurs on a OMA access, a unique
interrupt is generated to the interrupt controller.
NOTE
When external buffer memory is used, the M68000 bus arbitration
delay must be less than what would cause the SCC internal FIFOs
to overrun or underrun. This aspect is discussed in more detail in
4.5 SERIAL COMMUNICATIONS CONTROLLERS (SCCs) and in AP-
PENDIX A
sec
PERFORMANCE.
The one general-purpose IOMA controller can operate in different modes of
data transfer as programmed by the user. The IOMA is capable of transferring
data between any combination of memory and 1/0. In addition, data may be
transferred in either byte or word quantities, and the source and destination
addresses may be either odd or even.
MC68302 USER'S MANUAL
MOTOROLA
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