10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM
Datasheet
Chapter 5 System Description
5.1
Microcontroller Interface
The top halves of Figure 5.1 and Figure 5.2 illustrate typical COM20022I interfaces to the microcontrollers.
The interfaces consist of a 8-bit data bus, an address bus and a control bus. In order to support a wide
range of microcontrollers without requiring glue logic and without increasing the number of pins, the
COM20022I automatically detects and adapts to the type of microcontroller being used. Upon hardware
reset, the COM20022I first determines whether the read and write control signals are separate READ and
WRITE signals (like the 80XX) or DIRECTION and DATA STROBE (like the 68XX). To determine the type
of control signals, the device requires the software to execute at least one write access to external memory
before attempting to access the COM20022I. The device defaults to 80XX-like signals. Once the type of
control signals are determined, the COM20022I remains in this interface mode until the next hardware
reset occurs. The second determination the COM20022I makes is whether the bus is multiplexed or non-
multiplexed. To determine the type of bus, the device requires the software to write to an odd memory
location followed by a read from an odd location before attempting to access the COM20022I. The signal
on the A0 pin during the odd location access tells the COM20022I the type of bus. Since multiplexed
operation requires A0 to be active low, activity on the A0 line tells the COM20022I that the bus is non-
multiplexed. The device defaults to multiplexed operation. Both determinations may be made
simultaneously by performing a WRITE followed by a READ operation to an odd location within the
COM20022I Address space 20022 registers. Once the type of bus is determined, the COM20022I
remains in this interface mode until hardware reset occurs.
Whenever nCS and nRD are activated, the preset determinations are assumed as final and will not be
changed until hardware reset. Refer to Description of Pin Functions section for details on the related
signals. All accesses to the internal RAM and the internal registers are controlled by the COM20022I. The
internal RAM is accessed via a pointer-based scheme (refer to the Sequential Access Memory section),
and the internal registers are accessed via direct addressing. Many peripherals are not fast enough to take
advantage of high-speed microcontrollers. Since microcontrollers do not typically have READY inputs,
standard peripherals cannot extend cycles to extend the access time. The access time of the COM20022I,
on the other hand, is so fast that it does not need to limit the speed of the microcontroller. The
COM20022I is designed to be flexible so that it is independent of the microcontroller speed.
The COM20022I provides for no wait state arbitration via direct addressing to its internal registers and a
pointer based addressing scheme to access its internal RAM. The pointer may be used in auto-increment
mode for typical sequential buffer emptying or loading, or it can be taken out of auto-increment mode to
perform random accesses to the RAM. The data within the RAM is accessed through the data register.
Data being read is prefetched from memory and placed into the data register for the microcontroller to
read. It is important to notice that only by writing a new address pointer (writing to an address pointer low),
one obtains the contents of COM20022I internal RAM. Performing only read from the Data Register does
not load new data from the internal RAM. During a write operation, the data is stored in the data register
and then written into memory. Whenever the pointer is loaded for reads with a new value, data is
immediately prefetched to prepare for the first read operation.
Revision 09-27-07
Page 16
SMSC COM20022I
DATASHEET
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