5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
6.0 SYSTEM DESCRIPTION
6.1 Microcontroller Interface
The top halves of Figure 2 and Figure 3 illustrate typical COM20020I 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 COM20020I automatically detects and adapts to the
type of microcontroller being used. Upon hardware reset, the COM20020I 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 COM20020I. The device defaults to 80XX-like signals. Once the type
of control signals are determined, the COM20020I remains in this interface mode until the next hardware reset occurs.
The second determination the COM20020I 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 COM20020I. The signal on the A0 pin during the odd location access tells the
COM20020I the type of bus. Since multiplexed operation requires A0 to be active low, activity on the A0 line tells the
COM20020I 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 COM20020I
Address space 20020D registers. Once the type of bus is determined, the COM20020I 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 FOR TQFP section for details on the related signals. All
accesses to the internal RAM and the internal registers are controlled by the COM20020I. 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 COM20020I, on the other hand, is so fast that it does not need to limit the speed of the
microcontroller. The COM20020I is designed to be flexible so that it is independent of the microcontroller speed.
The COM20020I 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 COM20020I 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 3.3V
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Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
