10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
Chapter 6 Functional Description
6.1
Microsequencer
The COM20022I contains an internal microsequencer which performs all of the control operations
necessary to carry out the ARCNET protocol. It consists of a clock generator, a 544 x 8 ROM, a program
counter, two instruction registers, an instruction decoder, a no-op generator, jump logic, and
reconfiguration logic.
The COM20022I derives a 20 MHz and a 10 MHz clock from the output clock of the Clock Multiplier.
These clocks provide the rate at which the instructions are executed within the COM20022I. The 20 MHz
clock is the rate at which the program counter operates, while the 10 MHz clock is the rate at which the
instructions are executed. The microprogram is stored in the ROM and the instructions are fetched and
then placed into the instruction registers. One register holds the opcode, while the other holds the
immediate data. Once the instruction is fetched, it is decoded by the internal instruction decoder, at which
point the COM20022I proceeds to execute the instruction. When a no-op instruction is encountered, the
microsequencer enters a timed loop and the program counter is temporarily stopped until the loop is
complete. When a jump instruction is encountered, the program counter is loaded with the jump address
from the ROM. The COM20022I contains an internal reconfiguration timer which interrupts the
microsequencer if it has timed out. At this point the program counter is cleared and the MYRECON bit of
the Diagnostic Status Register is set.
Revision 02-27-06
Page 30
SMSC COM20022I
DATASHEET
SMSC [ SMSC CORPORATION ]
