
2-14
Memory Management
2.5
2.5.1
OTHER MEMORY CONTROLLER INFORMATION
ROM Chip-Select Command Gating
The DOSCS and ROMCS chip-select outputs of the élanSC310 microcontroller are, by
default, internally gated with the memory-read command (MEMR) or the memory-write
command (MEMW). In the élanSC310 microcontroller, the following configuration regis-
ter bits may be used to disable the command gating and allow the DOSCS and ROMCS
signals to be available as address decodes only:
I
Bit 2 of the Miscellaneous 5 register at Index B3h
an address decode as follows:
Enables the ROMCS signal as
— 0
Address decode with command gating
— 1
Address decode only
I
Bit 4 of the ROM Configuration 3 register at Index B8h
nal as an address decode as follows:
Enables the DOSCS sig-
— 0
Address decode with command gating
— 1
Address decode only
When the CPU clock is stopped, the ROMCS and DOSCS chip-selects are forced High.
2.5.2
Wait States and Command Delays
The élanSC310 microcontroller provides several programmable options for controlling the
number of wait states and command delays inserted into a cycle. This section covers wait
states and command delays for ROM-BIOS, ROM-DOS, and ISA cycles.
A command delay is inserted between the point in a memory or I/O cycle where the
address is placed on the bus and the point where the memory or I/O, read or write, com-
mand signal is asserted. This delay gives slower devices extra time to decode the
address. Command delays do not lengthen the overall cycle time. That is, the command
signal is deasserted at the same time it normally is (determined by the number of wait
states), without the command delay. The net effect is a shortened command time.
Table 2-8 on page 2-14 documents the duration of the command delay for the various
cycles.
Table 2-8
Command Delay Duration for Various Cycles
Cycle Type
Command Delay Duration (in SYSCLK Cycles)
8-bit ISA memory
1 or 0.5 (determined by bit 2 of the Command Delay register at
Index 60h)
8-bit ROM-DOS memory
0, 0.5, or 1 (determined by bit 2 of the Command Delay register and
bits 2 and 6 of the MMS Memory Wait State 2 register)
8-bit ROM-BIOS memory
0.5
16-bit memory (all)
0
8-bit external I/O (0100–03FFh)
0.5, 1, 2 (determined by bits 0 and 1 of the Command Delay register)
8-bit internal I/O (000–0FFh)
0.5
16-bit external I/O (0100–03FFh)
0.5