AVR based SCSI RAM disk

Overview

Because I am a SCSI fan, I want to have a SCSI controller to play with
...

The goal of this project was a universal SCSI controller (not a
hostadapter) consisting of standard parts. It should be modular,
SCSI3 (ANSI X3.301:1997) compliant and SE/HVD/LVD compatible with
corresponding PHYs.
I have chosen a RAM disk as logical unit to focus development on the
SCSI controller not to media access and/or OS drivers (RAM is easier to
control than other media and any OS already have drivers for Type 0
"direct access" devices).

The SCSI controller developed with this project I have used for my
SCSI PCMCIA drive.

Hardware

At the beginning I cannot get my hands on that nice SN75LVDM977 LVD/SE
multimode tranceivers from Texas Instruments so the prototype have a
SE PHY using bipolar TTL technology (robust, cheap and available).
Four 30Pin DRAM modules I found in my equipment become the media. Two
AVR microcontrollers do the work (one for the parallel SCSI interface,
the other for the target and logical unit functions).
The result was the working prototype in 3.5" floppy drive
formfactor shown below:

RAM disk prototype hardware V1.0 patched to V1.0.1 (click to enlarge)

I have designed the PCB with Eagle V4.09 for GNU/Linux from
Cadsoft Computer GmbH.
Two prototypes of the PCB were manufactured by
Beta Layout GmbH for me.
Because of some design flaws that firmware development have revealed,
I have to patch several things (the red wires in the picture) resulting
in Hardware V1.0.1:

Hardware V1.0.1 placeplan (click to enlarge)

The schematic and layout data can be downloaded here:

• Schematic V1.0.1 [Portable document] 2.5MiByte
  
• Eagle project V1.0.1 [gzip compressed tar archive] 1MiByte
  
  Note: The files in this archive are created with a legal copy of
  Eagle V4.09 (some libraries are modified versions of the ones
  shipped with different Eagle versions)
  

I claim only the right of being the originator of the schematic and
layout, feel free to use it for everything you want. Some libraries are
modified versions and originally created by Cadsoft. You can use the
modified libraries for any purpose (the terms of Cadsoft apply if I
overlooked something and this is not allowed).

Firmware

The "PIA" and "Target" modules requires firmware to
work. I have developed the firmware on a GNU/Linux host, it should be
possible to build it with the shipped Makefiles on any UNIX host if
the following tools are available:

• GCC V3.3 (C cross compiler for AVR)
• GNU binutils V2.14 (required by GCC)
• avr-libc V1.0 (required by GCC)
• tavrasm V1.22 (Macro assembler for AVR)
• My
  file format converters [optional]
  

Source code and images for Flash-EPROM and EEPROM are included in the packages.

Note: PIA and target firmware with equal major numbers are compatible.

PIA (Parallel Interface Agent) firmware

The PIA firmware is relatively small (about 3KiByte) and written in
assembler. The PIA controls the SCSI bus by implementing the SPI (SCSI
Parallel Interface) and provides a SDS (Service Delivery Subsystem) as
descibed in the SAM document to the Target module.

PIA firmware is available under
BSD license:

• PIA documentation [Portable document] 185KiByte
  
• PIA firmware V0.0 [gzip compressed tar archive] 67KiByte
  
• PIA firmware V0.1 [gzip compressed tar archive] 71KiByte
  
  Multibyte routines for SCSI data phases increase maximum transfer
  rate.
  

Target firmware

The Target firmware is written in C and about 24KiByte in size. With
enabled debug system the size increases to about 55KiByte (mainly due
to string constants and inlining). The Target module implements a
logical unit of Type 0 "direct access" that should comply to
the SPC and SBC documents (and the handling of INQUIRY and REQUEST
SENSE commands for nonexisting LUNs).

The logical unit uses four standard 30Pin memory modules with
asynchronous DRAM as media and map it to logical blocks of 512Byte
size.

Target firmware is available under
GNU general public license:

• Target documentation [Portable document] 2.2MiByte
  
  Target documentation [bzip2 compressed portable document] 91KiByte
  
• Target firmware V0.0 [gzip compressed tar archive] 631KiByte
  
• Target firmware V0.1 [gzip compressed tar archive] 631KiByte
  
  Bugfixes in PIA driver.
  

The target firmare contains a debug system that can print messages via
the AVRs USART to a terminal. A level converter like MAX232 must be
connected to CON30. The debug messages for different modules can be
separately enabled and disabled (the module is printed in brackets).

The Adaptec AHA-2940 device scan issues a SCSI bus reset and executes
an INQUIRY command. This is logged like this:

SCSI RAM-disk debug system
==========================

Hardware: V1.0.1, Firmware: V0.0 (Build: 2005-07-17 21:10:39 UTC)

Init target:
Activate UNIT ATTENTION condition
Enable interrupts ... OK
Enable DRAM refresh ... OK
Init DRAM ... OK

Target POST:
SRAM test ... OK
DRAM size ... 4MiByte
DRAM test ... OK
POST complete

Init PIA:
Read our SCSI ID from jumper block ... 6
Configure PIA for target mode ...
PIA has enabled the SCSI interface

Boot complete

Starting taskrouter
[Task router] Prepare for next task
[Task router] Idle
[Task router] An initiator selects us
[Task router] Connected to initiator with SCSI ID 7
[Task router] I_T nexus established
[Task router] ATTENTION condition detected
[Task router] Receive message from initiator
[Task router] Message: C0
[Task router] IDENTIFY message received
[Task router] LUN 0 selected
[Task router] I_T_L nexus established
[Task router] ATTENTION condition detected
[Task router] Receive message from initiator
[Task router] Message: 01 03 01
[Task router] Unknown message received
[Task router] Send MESSAGE REJECT message
[Task router] Receive command from initiator
[Task router] Check for reservation conflict
[LUN 0 Taskmanager] Check for reservation
[Task router] Create task for LUN 0
[LUN 0 Taskmanager] Create task
[LUN 0 Deviceserver] Execute task
[LUN 0 Deviceserver] CDB: 12 00 00 00 20 00
[LUN 0 Deviceserver] Command: INQUIRY
[LUN 0 Deviceserver] Command successfully completed
[Task router] Success, report status GOOD
[Task router] Send TASK COMPLETE message
[Task router] Release SCSI bus
[Task router] Prepare for next task
[Task router] Idle
      

The messages logged for a Linux boot that leads to this:

--------------------------------------------------------------------------------
(scsi0) Adaptec AHA-294X SCSI host adapter found at PCI 2/0
(scsi0) Narrow Channel, SCSI ID=7, 16/255 SCBs
(scsi0) Downloading sequencer code... 406 instructions downloaded
scsi0 : Adaptec AHA274x/284x/294x (EISA/VLB/PCI-Fast SCSI) 5.1.17/3.2.4
       [Adaptec AHA-294X SCSI host adapter]
scsi : 1 host.
(scsi0:0:0:0) Synchronous at 5.0 Mbyte/sec, offset 8.
  Vendor: QUANTUM   Model: MAVERICK 540S     Rev: 0901
  Type:   Direct-Access                      ANSI SCSI revision: 02
Detected scsi disk sda at scsi0, channel 0, id 0, lun 0
  Vendor: TOSHIBA   Model: CD-ROM XM-3301TA  Rev: 2411
  Type:   CD-ROM                             ANSI SCSI revision: 02
  Vendor: Selfmade  Model: RAM-disk          Rev: V1.0
  Type:   Direct-Access                      ANSI SCSI revision: 03
Detected scsi disk sdb at scsi0, channel 0, id 6, lun 0
scsi : detected 2 SCSI disks total.
SCSI device sda: hdwr sector= 512 bytes. Sectors= 1057758 [516 MB] [0.5 GB]
SCSI device sdb: hdwr sector= 512 bytes. Sectors= 8192 [4 MB] [0.0 GB]
Partition check:
 sda: sda1 sda2
 sdb: unknown partition table
--------------------------------------------------------------------------------
      

can be found here:
Debug messages for Linux boot [ASCII]

Ressources

• The great german book "SCSI Bus und IDE Schnittstelle" from
  Friedhelm Schmidt (Addison-Wesley)
  
  An english translation with the name "SCSI Bus and IDE
  Interface" is available
  
• T10 SCSI drafts (freely available)
  
  Relevant documents: SAM, SPI, SPC, SBC
  
• AVR-GCC FAQs