5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
High Speed CPU Bus Timing Support
High speed CPU bus support was added to the COM20020I. The reasoning behind this is as follows: With the Host
interface in Non-multiplexed Bus mode, I/O address and Chip Select signals must be stable before the read signal is
active and remain after the read signal is inactive. But the High Speed CPU bus timing doesn't adhere to these
timings. For example, a RISC type single chip microcontroller (like the HITACHI SuperH series) changes I/O address
at the same time as the read signal. Therefore, several external logic ICs would be required to connect to this
microcontroller.
In addition, the Diagnostic Status (DIAG) register is cleared automatically by reading itself. The internal DIAG register
read signal is generated by decoding the Address (A2-A0), Chip Select (nCS) and Read (nRD) signals. The decoder
will generate a noise spike at the above tight timing. The DIAG register is cleared by the spike signal without reading
itself. This is unexpected operation. Reading the internal RAM and Next Id Register have the same mechanism as
reading the DIAG register.
Therefore, the address decode and host interface mode blocks were modified to fit the above CPU interface to
support high speed CPU bus timing. In Intel CPU mode (nRD, nWR mode), 3 bit I/O address (A2-A0) and Chip
Select (nCS) are sampled internally by Flip-Flops on the falling edge of the internal delayed nRD signal. The internal
real read signal is the more delayed nRD signal. But the rising edge of nRD doesn't delay. By this modification, the
internal real address and Chip Select are stable while the internal real read signal is active. Refer to Figure 4 below.
VALID
A2-A0, nCS
nRD
Delayed nRD
(nRD1)
Sampled A2-A0, nCS
VALID
More delayed nRD
(nRD2)
FIGURE 4 - HIGH SPEED CPU BUS TIMING - INTEL CPU MODE
The I/O address and Chip Select signals, which are supplied to the data output logic, are not sampled. Also, the nRD
signal is not delayed, because the above sampling and delaying paths decrease the data access time of the read
cycle.
The above sampling and delaying signals are supplied to the Read Pulse Generation logic which generates the
clearing pulse for the Diagnostic register and generates the starting pulse of the RAM Arbitration. Typical delay time
between nRD and nRD1 is around 15nS and between nRD1 and nRD2 is around 10nS.
Longer pulse widths are needed due to these delays on nRD signal. However, the CPU can insert some wait cycles
to extend the width without any impact on performance.
The RBUSTMG bit was added to Disable/Enable the High Speed CPU Read function. It is defined as: RBUSTMG=0,
Disabled (Default); RBUSTMG=1, Enabled.
Revision 12-06-06
18
SMSC COM20020I 3.3V
DATASHEET
SMSC [ SMSC CORPORATION ]
