A specification for a Universal Spaceship Bus for the 0x10c DCPU

USSP_Spec.txt

The Universal Spaceship Bus is a time division multiplexed ship wide communications
medium. On startup each device is assigned an address by the bus master. The bus master
cycles through each device in turn asking it if it has a message to send. If the device
has a message to send, the buss master will obtain the destination address from the source
and then ask the destination device if it is ready to receive a message, if yes, then the
bus master will transfer the message from source to destination. If the destination device
isn't ready, the bus master will move on to the next device on the bus.

The exchange looks like this:

Each line represents one clock cycle

T  |SRC |     |Master|     | Dst | Comment
---+----+-----+------+-----+-----+-----
01 |    | <-- | RTS? |     |     | 
02 |ACK | --> |      |     |     | Or send NACK if not ready to send
03 |    |     | RTR? | --> |     | 
04 |    |     |      | <-- | ACK | Or send NACK if not ready to receive
05 |    | <-- | SND  |     |     | 
06 |HDR | --> |      |     |     | 
07 |PLD1| --> | HDR  | --> |     | 
08 |PLD2| --> | PLD1 | --> |     | 
09 |PLD3| --> | PLD2 | --> |     | 
10 |PLD4| --> | PLD3 | --> |     | 
11 |PLD5| --> | PLD4 | --> |     | 
12 |PLD6| --> | PLD5 | --> |     | 
13 |    |     | PLD6 | --> |     | 
14 |    |     |      | <-- | ACK | Or send NACK if message was corrupted
15 |    | <-- | ACK  |     |     | Or forward NACK

The message above are:
RTS = Are you ready to send
RTR = Are you ready to receive
ACK = Yes or Received
NACK = No or not received/send
HDR = Payload Header (The first 8 bits contain the payload type and command to
      destination) The second 8 bits are an 8 bit CRC that includes the first
      8 bits of the header plus the 4 payload words.
PLD1 = First payload word
PLD2 = Second payload word
PLD3 = Third payload word
PLD4 = Fourth payload word
PLD5 = Fourth payload word
PLD6 = Fourth payload word


If the destination answers NACK, or fails to reply, the master will send a NACK to the
source and move on to the next device.

The maximum message rate for the USSB is one message every 15 clock ticks. If the bus is
operating at 100KHz then the maximum message rate is 6666 messages per second or
106,666 bytes per second.

Most devices don't implement the bus protocol directly but instead use a bus interface card.
The bus interface card or BIC provides a simplified interface to the bus. The BIC allows
the device to place a single complete message in the BIC's transmit buffer that the BIC can
transmit at it's leisure. The BIC also buffers a single received message.

The BIC can notify the device of message transmission or reception via a pin assert (in the
case of an integrated circuit) or trigger an interrupt (in the case of a device card in a
computer system).

The message format that the BIC provides is:

+----------------+----------------+
|TTTTSSSSSSSSSSSS|CCCCDDDDDDDDDDDD|
+----------------+----------------+
|PPPPPPPPPPPPPPPP|PPPPPPPPPPPPPPPP|
+----------------+----------------+
|PPPPPPPPPPPPPPPP|PPPPPPPPPPPPPPPP|
+----------------+----------------+
|PPPPPPPPPPPPPPPP|PPPPPPPPPPPPPPPP|
+----------------+----------------+
|
| T = Payload Type
| S = Source Address
| C = Device specific Command
| D = Destination Address
| P = Payload
|
+---------------------------------

The first word contains the 12 bit source address and a 4 bit payload type code. The
payload type code is device specific but two codes are standardized. A value of 0
indicated that the payload is empty and a value of 1 indicates that the payload contains
the device information.

The second word contains the 12 bit destination address and a 4 bit command code. The
command code is device specific but two values are standardized. A value of 0 indicates
a device probe and a value of 1 indicates a device probe reply. A device is expected to
reply to every device probe with a device information payload.

The device information payload is formatted as:

+----------------+----------------+
|HHHHHHHHHHHHHHHH|HHHHHHHHHHHHHHHH|
+----------------+----------------+
|MMMMMMMMMMMMMMMM|MMMMMMMMMMMMMMMM|
+----------------+----------------+
|VVVVVVVVVVVVVVVV|DDDDDDDDDDDDDDDD|
+----------------+----------------+
|
| H = Hardware Id (low word first)
| M = Hardware Manufacturer (low word first)
| V = Hardware Version
| D = Additional device specific information
|
+---------------------------------

The BIC interface spec for the DCPU might look like this:

Name: USSB Interface Card
ID: TBD
Version: 1

When the device receives an interrupt it will:

 A | BEHAVIOR
---+----------------------------------------------------------------------------
 0 | Reset the BIC. All buffers are cleared, status word is set to 0
 1 | Place the BIC's status word into B
   |    Bit 0 is the RTS bit (0 = nothing to send, 1 = ready to send)
   |    Bit 1 is the RTR bit (0 = not ready to receive, 1 = ready to receive)
   |    Bits 8-11 are the send error code
   |    Bits 12-15 are the receive error code
 2 | Take the 8 words from RAM starting at the address indicated by register B
   |   and place them in the transmit buffer. This will overwrite any values
   |   currently in the transmit buffer. RTS set from register C
 3 | Take the 8 words from the receive buffer and write them to the RAM address
   |   indicated by register B.  RTR set from register C
---+----------------------------------------------------------------------------

When interrupts are enabled and the device sends an interrupt:

 A | BEHAVIOR
---+----------------------------------------------------------------------------
 0 | An error has occurred. BIC Status word is placed in B
 1 | Message was transmitted, RTS bit is cleared.
 2 | Message has been received, RTR bit is cleared.
---+----------------------------------------------------------------------------

The BIC Send error codes are:
0x00 = Last transmit succeeded
0x01 = Last transmit failed because device never replied
0x02 = Last transmit failed because device NACK'ed the RTR
0x03 = Last transmit failed because device NACK'ed the message
0x04 = Last transmit failed because the master detected an error in the CRC

The BIC recieve error codes are:
0x00 = The last received was a success
0x01 = Received an RTR, but didn't receive HDR
0x02 = Received full message but had to send NACK due to CRC error