5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 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 COM20020ID 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 COM20020ID automatically detects and adapts to the type of microcontroller being used. Upon
hardware reset, the COM20020ID 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 COM20020ID. The device defaults to 80XX-like
signals. Once the type of control signals are determined, the COM20020ID remains in this interface mode
until the next hardware reset occurs. The second determination the COM20020ID 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
COM20020ID. The signal on the A0 pin during the odd location access tells the COM20020ID the type of
bus. Since multiplexed operation requires A0 to be active low, activity on the A0 line tells the
COM20020ID 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 COM20020ID Address space 20020D registers.
Once the type of bus is
determined, the COM20020ID 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 COM20020ID.
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 COM20020ID, on the other hand, is so fast that it does not need to limit the speed of the
microcontroller. The COM20020ID is designed to be flexible so that it is independent of the microcontroller
speed.
The COM20020ID 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 COM20020ID 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.
SMSC COM20020I Rev D
Page 17
Revision 12-05-06
DATASHEET
SMSC [ SMSC CORPORATION ]
