SSI

Serial Synchronous Interface

Overview

The definition of the SSI is in XMM-OM/MSSL/SP/0007 "Electrical Interfaces Specification".

Hardware

The interface consists of:

• SSI_CLK: a continuous clock signal generated by the ICU
• SSI_ENV_TX: active high when data present
• SSI_DATA_TX: 16-bit data
• SSI_ENV_RX: active high when data present
• SSI_DATA_RX: 16-bit data
• Signal return

Commands are sent from the ICU to the DPU. Science data is passed from the DPU to the ICU when demanded by the ICU. Alerts are sent (unrequested) by the DPU to the ICU. There is no direct feedback as part of the protocol and there is no error correction nor checksums. The interface can be thought of as the same irrespective of direction.

The SSI clock frequency is 125 kHz producing a period of 8 us (1 bit-period). The SSI 16-bit data words are separated by at least one bit-period and at most the SSI block gap (defined in software). The SSI data blocks are separated by at least the SSI block gap (defined in software).

Transmitting data

Receiving data

SSI block gaps

1. 16-bit type - the block type
2. 16-bit length - the number of 16-bit words following this word (i.e. total length - 2)
3. the rest of the data

Regular DPU to ICU data blocks

Regular science data.

DPU priority data

These contain science data that is sent out as soon as it is available rather than at the end of an exposure.

DPU RAM dumps

RAM dumps.

DPU to ICU alerts

Alerts from the DPU to signify something is has happened, is ready or an error has occured.

ICU to DPU commands

Commands to the DPU.

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Further detail on the ICU software

The first, fast part of the SSI interrupt handler is written in assembler (the first word of the SSI block is read) and the rest in written in Ada (the reading of the rest of the words in the block and the timeout.)

SSI status register

D_TX	2**4
DATA_FULL	2**3
OVF	2**2
D_RX	2**1
INT	2**0

Sequence of actions

• SSI INTERRUPT happens
• Read first word (from i/o address f241h) into input software fifo in less than 136 us after the interrupt
• Remember location where next word will be stored for a later check
• Start stopwatch
• Set interrupt mask to only allow RBI interrupts
• Enable interrupts but don't get interrupted for too long!
• loop
  • read SSI status (i/o address f240h)
  • if the DATA_FULL bit (2**3) is set and there is data to output
    • write a data word to output i/o address (7241h)
  • if input software fifo is full
    • error
  • if D_RX bit is reset
    • read input word (i/o address f241h) into input software fifo
    • re-start stopwatch because there is still data on input
  • else
    • if stopwatch is after 4 ms
      • break out of loop
  • read ssi status word (i/o address f240h)
  • if OVF bit (2**2) is 0
    • clear overflow (write fffb (hex) to status register i/o address 7240h)
    • read a word (from i/o address f241h) and dispose of it
• end loop
• read the second word (length) of this SSI block from the software input buffer
• if it is greater then 1027
  • error
• if no of words read doesn't equal the value of the second word (see above) minus 2
  • error
• read ssi status word (i/o address f240h)
• if OVF bit (2**2) is 0
  • clear overflow (write fffb (hex) to status register i/o address 7240h)
  • read a word (from i/o address f241h) and dispose of it
• clear SSI interrupt by writing fffe (hex) to the SSI status i/o address 7240h

To Reset

• reset software input and output fifos and error value
• write OVR_WR fffb (hex) to status address 7240 (hex)
• write INT_WR fffe (hex) to status address 7240 (hex)

SSI error codes

error = C

The SSI input circular buffer has filled so fast or not been emptied fast enough and incoming data is about to overwrite outgoing data.

error = 2

The word count is too large while receiving data in the block. The number of words has exceeded that indicated by the second "block length" word or has exceeded the maximum allowed (1029).

error = 8

An overflow (OVF) has been indicated by the ICU's SSI hardware.

error = 7

An overflow occured at the end of the block.

error = 11

The second word of the block indicated a length which exceeds the maximum allowed (1029).

error = 1

The length indicated by the second word is inconsistent with the real length of the block.

error = 89

An overflow was found during SSI_DRIVER.PUT

error = 9

The length found in SSI_DRIVER.PUT exceeded the maximum allowed (1029).

error = b

The output block length in SSI_DRIVER.PUT exceeded the maximum allowed (1029).

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Further detail on the DPU software

The DSP converts a serial SSI word to parallel word. Each received word generates an interrupt. The SSI ISR pushes the word into a circular buffer. The 1ms ISR checks the COLLECTING_A_COMMAND bit. If it is zero (cleared), it decrementes the delay count (stopwatch), else the delaycount (stopwatch) is reset. When the delaycount reaches 0, it is assumed a valid comand has been received (a full block has been received), and the command interpreter is called. The command interpreter checks for integrity of command: it checks the block has:

• a valid command ID
• a legal length for command ID

It does not count the number of words received and compare this with the length stored as the second word. The command interpreter is written in C and the rest of the SSI code in assembler.

On a hardware error the code will:

• Reset fill pointer.
• Send out bad block.

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