MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 468

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

Design Checklist
F-4
23. SCCs, Diagnostics
24. UART, MAX_IDL
25. Write Zeroes into Reserved Locations
26. Event Registers, Read-Modify-Write
27. Microcode, RAM
When an unexplained SCC problem arises, three important debugging techniques can
be used to help isolate the problem area. First, to see if the problem is related to the
external M68000 bus, try configuring the transmit and/or receive buffers to be located
in the internal dual-port RAM. In this way, the external bus will not be required for
transmission or reception. Second, to see if the problem is related to the serial inter-
face pins, try setting the DIAG1–DIAG0 bits in the SCM register to loopback mode or
software operation. These techniques allow transmission and reception to be tested
without proper settings of the serial interface control pins. Loopback mode is also a
good test mechanism to build into the application board test code. Third, to see if the
problem is related to rise times, noise, or glitches on external serial clocks, try using
the SCC internal baud rate generator or try generating a clock externally with another
baud rate generator or timer and routing that clock externally to the SCC.
One common problem with UART mode is the setting of the MAX_IDL value. The max-
imum value is $0000, and the minimum value is $0001. A large value causes the re-
ceive buffer not to close until the entire idle period is complete. At slow serial data
rates, this time can be several minutes.
Zero fill all reserved locations to prevent unexpected behavior.
Do not use instructions that execute a read-modify-write cycle (e.g., ANDI, ORI, BSET,
BCLR) to clear bits in the event registers. A bit in such a register is cleared by writing
a one, rather than a zero, to the bit. The use of instructions that execute a read-modify-
write cycle inadvertently clears all bits in the event register. The clearing of all bits in
an event register could result in “lost” events. For example, when an interrupt handler
intentionally or unintentionally clears all bits of an event register upon exit of the han-
dler, all events occurring between the time of the entrance and exit of the interrupt han-
dler are lost. A proper handling of event registers is to clear particular bits near the
entrance of an interrupt handler. A proper instruction to use is the MOVE instruction
(e.g., “MOVE #$01, SCCE” clears bit 0 of the SCCE register).
If microcode from RAM is used (i.e., one of the packages available from Motorola), the
microcode must be downloaded into the MC68302 dual-port RAM prior to setting lo-
cation $0F8 to $0001. If this sequence is not followed, the CP will not function properly,
regardless of which protocol is selected.
MC68360 USER’S MANUAL
MOTOROLA