COM20020I_0609 Datasheet, PDF (65/69 Pages) SMSC Corporation – 5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Chapter 10 Appendix A
This appendix describes the function of the NOSYNC and EF bits.
NOSYNC Bit
The NOSYNC bit controls whether or not the RAM initialization sequence requires the line to be idle by enabling or
disabling the SYNC command during initialization. It is defined as follows:
NOSYNC: Enable/Disable SYNC command during initialization. NOSYNC=0, Enable (Default): the line has to be idle
for the RAM initialization sequence to be written, NOSYNC=1, Disable: the line does not have to be idle for the RAM
initialization sequence to be written.
The following discussion describes the function of this bit:
During initialization, after the CPU writes the Node ID, the COM20020I 3V will write "D1"h data to Address 000h and
Node-ID to Address 001h of its internal RAM within 6uS. These values are read as part of the diagnostic test. If the D1
and Node-ID initialization sequence cannot be read, the initialization routine will report it as a device diagnostic failure.
These writes are controlled by a micro-program which sometimes waits if the line is active; SYNC is the micro-program
command that causes the wait. When the micro-program waits, the initial RAM write does not occur, which causes the
diagnostic error. Thus in this case, if the line is not idle, the initialization sequence may not be written, which will be
reported as a device diagnostic failure.
However, the initialization sequence and diagnostics of the COM20020I 3V should be independent of the network
status. This is accomplished through some additional logic to decode the program counter, enabled by the NOSYNC
bit. When it finds that the micro-program is in the initialization routine, it disables the SYNC command. In this case,
the initialization will not be held up by the line status.
Thus, by setting the NOSYNC bit, the line does not have to be idle for the RAM initialization sequence to be written.
EF Bit
The EF bit controls several modifications to internal operation timing and logic. It is defined as follows:
EF: Enable/Disable the new internal operation timing and logic refinements. EF=0: (Default) Disable the new internal
operation timing (the timing is the same as in the COM20020I Rev. B); EF=1: Enable the new internal operation timing.
The EF bit controls the following timing/logic refinements in the COM20020I 3V:
A) Extend Interrupt Disable Time
While the interrupt is active (nINTR pin=0), the interrupt is disabled by writing the Clear Tx/Rx interrupt and Clear Flag
command and by reading the Next-ID register. This minimum disable time is changed by the Data Rate. For
example, it is 200 nS at 2.5 Mbps and 100 nS at 5 Mbps. The 100 nS width will be too short to for the Interrupt to be
seen.
Setting the EF bit will change the minimum disable time to always be more than 200 nS even if the Data Rate is 5
Mbps . This is done by changing the clock which is supplied to the Interrupt Disable logic. The frequency of this clock
is always less than 20MHz even if the data rate is 5 Mbps.
B) Synchronize the Pre-Scalar Output
The Pre-Scalar is used to change the data rate. The output clock is selected by CKP3-1 bits in the Set-Up register. The
CKP3-1 bits are changed by writing the Set-Up register from outside the CPU. It's not synchronized between the CPU
and COM20020I 3V. Thus, changing the CKP3-1 timing does not synchronize with the internal clocks of Pre-Scalar,
and changing CKP3-1 may cause spike noise to appear on the output clock line.
Setting the EF bit will include flip-flops inserted between the Configuration register and Pre-Scalar for synchronizing
the CKP3-1 with Pre-Scalar’s internal clocks.
SMSC COM20020I 3.3V Rev.E
Page 65
DATASHEET
Revision 09-11-06