5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
5.1.1 High Speed CPU Bus Timing Support
High speed CPU bus support was added to the COM20020ID. 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 SH-1 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 5.3 below.
VALID
A2-A0, nCS
nRD
Delayed nRD
(nRD1)
Sampled A2-A0, nCS
VALID
More delayed nRD
(nRD2)
Figure 5.3 – 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 12-05-06
Page 20
SMSC COM20020I Rev D
DATASHEET
SMSC [ SMSC CORPORATION ]
