MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 467

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

MOTOROLA
17. Function Code, lnitialize, Reset
18. Function Code, External Bus Master
19. BAR, Write
20. DRAM Refresh
21. Watchdog Timer
22. Underrun, Overrun, Clock Lines, Schmitt-Triggers
does not correctly “equate” locations in parameter RAM to intended addresses. A sim-
ple typo in an assembler EQU or a C #DEFlNE directive can cause 1) the intended
parameter not to be set and 2) another parameter to be set to a wrong value.
To use IDMA, SDMA, and/or DRAM refresh, their corresponding function code regis-
ters MUST be initialized. Setting the function codes in these registers to “111” will pre-
vent the MC68302 chip selects from asserting. Failure to initialize these registers often
results in their function codes having the value “111”, since these registers are in dual-
port RAM and do not have predefined values upon a total system reset.
When an external bus master is using the chip selects on the MC68302 with the ex-
ternal master's external memory accesses, make sure that the external bus master
drives the function code lines to something other than “111". If the function code lines
are driven or left floating to “111”, the external cycle will be interpreted by the MC68302
as an interrupt acknowledge cycle, and the chip selects will not be asserted during the
cycle.
The BAR MUST BE written by an instruction following a total system reset of the
MC68302 since this register resides in the MC68302, not in the memory. It is not suf-
ficient or required for the EPROM on the target board to have the desired BAR value
stored in the EPROM location $0F2 (the address of BAR). When using an emulator, a
symptom of this problem can be that the code works in the emulator overlay memory,
but not on the target.
When using the DRAM refresh unit, one cannot refresh locations $0F0–0FF of an ex-
ternal DRAM if an MC68302 chip select is used to select that DRAM. Locations $0F0–
0FF are designated as the reserved area of the IMP that contains the BAR and SCR,
and chip selects will not activate on accesses to these addresses. The remedy is sim-
ply to use a different DRAM refresh starting address besides $0. Also note that the
DRAM refresh access is a byte read, not a word read.
If the MC68302 watchdog timer is never turned off or refreshed, an unexpected inter-
rupt at level 4 can occur. Also, an unexpected RESET can occur if the WDOG pin is
externally connected to HALT and RESET. The solution is to disable the watchdog tim-
er after reset. Note that the watchdog timer is not related to the hardware watchdog,
which is a completely separate unit that monitors bus activity.
If a transmit underrun or a receive overrun is reported but the data rates are too slow
to suggest an actual underrun or overrun, the problem may be in the clock lines.
Glitched or badly ringing clocks (on the TCLK or RCLK pins) can cause SCCs to enter
either of the above error states. Even though Schmitt-triggers are implemented on the
IMP clocks lines, a very slow rise/fall time coupled with a large amount of noise on the
lines can override the hysteresis protection and affect the ability of the SCC to correctly
sample and clock data. Internal clocks generated by the IMP do not cause this prob-
lem.
MC68360 USER’S MANUAL
Design Checklist
F-3