80C186EC/80C188EC Microprocessor User's Manual

CHAPTER 6
CHIP-SELECT UNIT
Every system requires some form of component-selection mechanism to enable the CPU to access
a specific memory or peripheral device. The signal that selects the memory or peripheral device
is referred to as a chip-select. Besides selecting a specific device, each chip-select can be
used to control the number of wait states inserted into the bus cycle. Devices that are too slow to
keep up with the maximum bus bandwidth can use wait states to slow the bus down.
6.1 COMMON METHODS FOR GENERATING CHIP-SELECTS
One method of generating chip-selects uses latched address signals directly. An example interface
is shown in Figure 6-1(A). In the example, an inverted A16 is connected to an SRAM device
with an active-low chip-select. Any bus cycle with an address between 10000H and 1FFFFH
(A16 = 1) enables the SRAM device. Also note that any bus cycle with an address starting at
30000H, 50000H, 70000H and so on also selects the SRAM device.
Decoding more address bits solves the problem of a chip-select being active over multiple address
ranges. In Figure 6-1(B), a one-of-eight decoder is connected to the uppermost address bits. Each
decoded output is active for one-eighth of the 1 Mbyte address space. However, each chip-select
has a fixed starting address and range. Future system memory changes could require circuit
changes to accommodate the additional memory.
6.2 CHIP-SELECT UNIT FEATURES AND BENEFITS
The Chip-Select Unit overcomes limitations of the designs shown in Figure 6-1 and has the following
features:
• Ten chip-select outputs
• Programmable start and stop addresses
• Memory or I/O bus cycle decoder
• Programmable wait-state generator
• Provision to disable a chip-select
• Provision to override bus ready
Figure 6-2 illustrates the logic blocks that generate a chip-select. Each chip-select has a duplicate
set of logic.
6-1