MCS9845 MosChip Semiconductor, MCS9845 Datasheet - Page 23

• Manufacturer Part Number: MCS9845
• Description: PCI Dual UART
• Manufacturer: MosChip Semiconductor

Rev. 2.5
ISA Bridge
A PCI to ISA Bridge allows the product designer to
increase the number of I/O Ports through the use of
external components. Additional UARTs and Parallel
Ports are easy to attach and confi gure.
Chip Selects
Four external Chip Select signals (nCS3-nCS6) are
provided. Each Chip Select has its own Base Address
Register (BAR) in the PCI Confi guration Space. The
Base Address Registers each point to a block of eight
I/O registers. When any of the eight registers in the
block assigned to one of the Base Address Registers
are accessed, the Chip Select signal for that group
will be activated.
relationship between the Base Address Registers and
the external Chip Select signals.
If BAR-3 contains the address 0xD800, then accessing
any address between 0xD800 and 0xD807 will activate
the nCS4 Chip Select line.
Common I/O Control Signals
In addition to the Chip Select signals, the other
required ISA signals are also present. Separate nIOR
(I/O Read) and nIOW (I/O Write) signals are provided,
simplifying the interface to external components. An
8-bit bi-directional Data Bus is provided. Both Active-
High and Active-Low Reset signals are provided,
simplifying the connection to external devices. Three
Address Lines (A0-2) allow each Chip Select to control
eight registers.
2 (E3)
3 (E4)
4 (E5)
5 (E6)
BAR
Offset
00-07
00-07
00-07
00-07
The following table shows the
Chip Select
nCS3
nCS4
nCS5
nCS6
Interrupts
ISA style devices typically use Active-High Interrupt
Request signals. Four Active-High Interrupt Request
inputs (IRQ2-5) are provided. A single Active-Low
Interrupt Request input (nIRQ6) is also available
providing even more fl exibility. There is no mechanism
provided to directly associate any of the Interrupt
Request signals with any of the external Chip Select
signals. The Interrupt Requests are simply passed on
to the PCI system. All I/O Ports or devices (including
those inside the chip) share a single PCI Interrupt
Request line. The software Driver must poll all of the
devices to determine which one is the source of an
interrupt.
Other Devices
While the primary purpose of the ISA Bridge is to allow
products to provide additional UARTs or Parallel Ports,
it is not necessarily restricted to such devices. The
signals provided should allow the inclusion of most ISA
style components into a custom design. If custom (or
other standard ISA) components are used, a custom
software Driver will probably be required as well. The
MosChip Drivers only support additional UARTs and
Parallel Ports. Simple custom devices that always
have the CPU initiate reads or writes to the device may
work without any additional Drivers, but any design
that adds external components that generate Interrupt
Requests will defi nitely require custom Drivers.
PCI Dual UART with ISA Bridge
MCS9845
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