10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM
Datasheet
The following sequences show the data transfer for a DMA read and a DMA write. The transfer of data
between system memory and internal RAM functions as a memory to I/O DMA transfer. Since it is treated
as an I/O device, the COM20022I has to create the RAM address. Therefore the COM20022I’s address
pointers must be programmed before starting the DMA transfers.
5.1.4 DMA Data Transfer Sequence (I/O to Memory: Read A Packet)
step1: Set DMA-controller (ex. 8237)
step2: Set DRQPOL, TCPOL, DMAMD1 and DMAMD0 bits
>>Finished DMA SETUP
>>A packet received
step3: Set address, byte count and etc. of DMA controller
step4: Set pointer High and Low (RDDATA=1,AUTOINC=1, DMAEN=0)
step5: Read SID, DID, CP in the received packet
step6: Set DMAEN=1 (RDDATA=1, AUTOINC=1)
step7: DMAEND=1 in Mask REG.
step8: Set pointer = CP
>>DREQ is asserted by step8
>>Interrupt occurs upon finishing DMA
5.1.5 DMA Data Transfer Sequence (Memory to I/O: Write A Packet)
step1: Set DMA-controller (ex. 8237)
step2: Set DRQPOL, TCPOL, DMAMD1 and DMAMD0 bits
>>Finished DMA SETUP
step3: Set address, byte count and etc. of DMA controller
step4: Set pointer High and Low (RDDATA=0,AUTOINC=1, DMAEN = 0)
step5: Write SID,DID,CP in the sending packet
step6: Set DMAEN=1 (RDDATA=0, AUTOINC=1)
step7: DMAEND=1 in Mask REG.
step8: Set pointer = CP
>>DREQ is asserted by step8
>>Interrupt occurs upon finishing DMA transfer
step9: Write Enable Transmit command to command register
5.1.6 High Speed CPU Bus Timing Support
High speed CPU bus support was added to the COM20022I. 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 5.7 on the following page.
Revision 09-27-07
Page 24
SMSC COM20022I
DATASHEET
SMSC [ SMSC CORPORATION ]
