Sysclk LWLA1034/Protocol

FPGA Configuration

The FPGA bitstream is loaded via bulk transfer to USB end point 4. Each firmware transfer starts with a 4-byte header to announce the transfer size. The payload appears to be a Raw Binary File (.rbf) with compression enabled.

Length	Payload...
nnnn-nnnn	dd...

Unlike the control commands, the firmware transfer is apparently byte-based. The length is a byte count encoded in big endian (1-2-3-4) byte order, and includes the size of the length field (4 bytes) itself.

Application Behavior

The vendor software transfers a new bitstream to the FPGA

1. on application start,
2. when switching clocking mode between internal, external/rising or external/falling,
3. on application exit.

The size of the bitstream varies due to compression, but is in the order of 50kB to 80kB. The firmware transfer is split by the vendor software into packets with 15 byte payload each and thus takes quite a bit of time inside VirtualBox. Testing with a libusb-based tool for issuing USB bulk transfers has shown that this does not appear to be necessary: Transferring even the entire firmware blob in a single bulk transfer appears to work without error.

Firmware Extraction

The firmware blobs can be extracted directly from the Windows installer executable located on the CD-ROM that ships with the device. The file lwla1034_EN_setup.exe on the CD-ROM from 2012-07-12 has the firmware blobs located at the following offsets:

Offset	Length	Mode
34110338	78398	Internal clock
34266237	78247	External clock (rising edge)
34344484	79145	External clock (falling edge)
34578631	48525	Shutdown

Both offsets and lengths are in bytes. The extracted blobs already include the header with the 32-bit length field.

Control Commands

Control commands are sent via bulk transfer to USB end point 2, with the response (if any) coming in from end point 6.

Command messages are sent to the device as a sequence of 16-bit words with little endian byte order. The first word in a message identifies the command type. Different command types have different message lengths. Some command types include a length field and allow for messages of variable length, others are of fixed size.

There are read commands which trigger an immediate response from the device, and write commands without a response.

Command 0001: Read Register

This command reads a 32-bit wide control register.

Command

Fixed length of 2 words (4 bytes).

ID	Address
0001	aaaa

Response

The response has a fixed length of 2 words (4 bytes). It is the content of a 32-bit register in mixed endian (2-1-4-3) byte order.

Registers

Address	Value
10C0	Channel 1 frequency counter
10C4	Channel 2 frequency counter
10C8	Channel 3 frequency counter
10CC	Channel 4 frequency counter

These registers apparently count the number of rising (or falling?) clock edges on channels 1 to 4. The vendor software polls these counters every second to display a live frequency count for the first four channels.

It is unclear what time base is being used for those counters: The large error (sometimes by more than 20%, especially for CH1) hints at I/O latency, which would imply that the software resets the counters. However, manual testing of single register reads has shown that the values do not seem to scale with the time between reads. This would mean the device is using an internal time base after all. However, that makes the large error a bit hard to explain, especially since the error is different for each channel despite being driven by the same signal source.

Address	Selftest value
10B0	12345678
10BC	12345678
10B8	87654321

These three registers assume the values listed here after writing 100 (decimal) to register 10B4. This is apparently an identification and self-test sequence, which is performed twice in a row during initialization. After initialization, these registers are part of command sequences which initiate device operations, like capture start/stop.

Address	Value
1078	Capture size (number of 36-bit words)

This register holds the (compressed) size of the captured data. The vendor software retrieves this just before fetching the SRAM content.

Command 0002: Write Register

This command writes a 32-bit value to a control register.

Command

Fixed length of 4 words (8 bytes).

ID	Address	Data
0002	aaaa	dddd-dddd

The value is encoded in mixed endian (2-1-4-3) byte order.

Registers

Address	Init value
10B4	00000064

The vendor software writes the value 100 (decimal) to this register during initialization into order to start the self test sequence. After that, the magic register values listed in the table for command 0001 become available. So, this is kind of a command-triggering register, and there are other command codes, too.

Address	Purpose
1094	Bypass clock divider
10B4	???
10B8	???
10BC	???

These are written to when a capture operation is initiated or cancelled. Register 1094 is set to 0 when using the internal clock with sampling rates of 100 MHz and below. For 125 MHz internal clock or the external clock modes, it is set to 1.

Address	Init value
1074	00000002
107C	00000000

The vendor software writes these two registers during initialization, and also when initiating a capture.

Command 0005: Write ???

This command appears to write 16 32-bit words at once. The vendor software issues this after a transfer of captured data to the host has finished, but only for the first three captures. After that, the application needs to be restarted to ever see this command again.

The data written varies each time, but it appears that only the lowest 8 bit of each 32-bit word are ever written. No idea what this might do. It could be calibration feedback or something, although it is hard to imagine how the host could help the device with that. Luckily, the sigrok driver works without ever sending this command at all.

Command

Fixed length of 33 words (66 bytes).

ID	Data 1	Data 2	...	Data 16
0005	dddd-dddd	dddd-dddd	...	dddd-dddd

The byte order of each 32-bit word is most likely the usual 2-1-4-3 mixed endianess.

Sample capture

A sample of three such commands from an actual protocol analysis session:

10185	56.455640		Cmd 5: write ??? data C3 9A 4B 91 30 D2 98 C1 CA F2 D7 4F 82 74 C0 8B
14155	95.788766		Cmd 5: write ??? data E5 7B 2C F3 6B 06 76 4B 60 94 F3 EC C9 AA E9 35
17823	131.101569		Cmd 5: write ??? data 16 66 EA 7B 35 F6 71 A8 0A E0 D7 E7 45 E0 4F 23

Only the least significant byte is shown for each 32-bit data word, as the other bytes are always 0. The message content changes with each run of the vendor software. It's hard to make head or tail of this. However, as this command is issued after all captured data has been retrieved, it cannot possibly affect the preceding capture and read-out operations.

Command 0006: Read Memory at Address

This command reads a chunk of data from the device memory (SRAM). It allows for random access using a 32-bit start address, and for variable length via a 32-bit length field. The software uses this command to read captured data from the device's buffer.

Command

Fixed length of 5 words (10 bytes).

ID	Address	Length
0006	aaaa-aaaa	nnnn-nnnn

Both the address and the length are encoded in mixed endian (2-1-4-3) byte order.

Response

The memory is 36 bit wide, and thus the size of the response in bits is 36 times the value in the length field. The original vendor software reads chunks of 120 words @ 36 bit at a time, which works out to an integer multiple of 32 (i.e. 4320 bits = 135 32-bit words or 540 bytes). The final six reads are done in chunks of 8 words @ 36 bit, which works out to nine 32-bit words or 36 bytes. The overall amount of memory being read when fetching captured samples from a full buffer is just below the RAM size of 256k×36 bit.

Note that the software always starts reading at address 4 rather than 0. Presumably, the firmware uses the first four 36-bit words for internal bookkeeping or some other purpose. Exception to the rule: For some unknown reason (perhaps testing), the memory is also being read on start-up right after loading the firmware into the FPGA. In this case, altogether 128000 36-bit words are being read beginning at address 0.

Note that reading more than 1024 bytes at a time seems to be unreliable. Due to the constraints outlined in the following section, the maximum read length should therefore be restricted to 224 device words, which works out to 1008 bytes.

36-to-32 Bit Mapping

The data returned by the read command consists of 32-bit words in 2-1-4-3 mixed endian byte order. Eight consecutive 36-bit words from the SRAM are mapped at a time to nine consecutive 32-bit words in the received stream. The first of these slices aligns to the beginning of the read-out stream, apparently irrespective of the absolute start address of the read operation.

The first eight 32-bit words in a slice contain the lower 32 bit of the eight encoded 36-bit words. The ninth 32-bit word contains the four remaining high bits of all eight 36-bit words combined. The high nibbles are shifted into the ninth word from right to left, resulting in a 1-2-3-4-5-6-7-8 order of nibbles. (This is after conversion from mixed endian byte order!)

As it is necessary to access the ninth 32-bit word in each slice even to fully extract the first 36-bit word, it follows that memory reads should always request a multiple of the slice length, i.e. eight 36-bit words. The length of the response will thus always be a multiple of nine 32-bit words (36 bytes). However, it does not appear to be necessary to restrict read operations to only the two lengths 120 and 8 used by the vendor software.

Compression Scheme

The compression scheme is a form of run-length encoding. Very short run lengths of only one or two cycles are handled as special cases, which helps to keep the worst-case overhead of compression pretty low. In particular, the scheme ensures that it is never necessary to write more than one 36-bit word per sampling cycle to the SRAM buffer.

Each 36-bit word in the stream is either a data word or a repeat half-count word. The first word in the stream is a data word, with the following layout:

Bit	35	34	33	32	31	...	2	1	0
Meaning	Repeat count follows	LSB of repeat count	CH34	CH33	CH32	...	CH3	CH2	CH1

If bit 35 is set, then the next 36-bit word in the stream encodes the number of cycles the previous data word is repeated, divided by two. The actual number of repeat cycles is twice that number plus the LSB of repeat count, i.e. bit 34 from the data word. If bit 35 is not set the next word is again a data word and the repeat half-count is assumed as zero. However, the LSB of repeat count bit still applies; i.e. if it is set then the repeat count would be 1.

So, to recap, repeat counts 0 to 1 are encoded as part of the channel data word, larger repeat counts use up a full extra 36-bit word. The combined repeat count is then 37 bit wide. At 125 MHz, this scheme would allow for encoding run lengths of more than 18 minutes. However, note that the vendor software does not actually make use of the full range: Apparently, it stops as soon as the count rolls over into bit 36, thereby cutting the maximum possible run length in half (i.e. about 9 minutes at 125 MHz). However, this detail does not make any difference for the decompression algorithm.

The next word following a repeat half-count is again a data word. Note that it is possible for a sample/run-length pair to be split across a slice boundary, or even across successive read chunks. The decoder therefore needs to keep track of RLE state across slices as well as read operations.

Command 0007: Capture Setup

This command prepares a capture operation. Essentially, this command appears to write to an internal memory block with 64-bit granularity. Command 0008 can be used to read from the same memory.

The vendor software issues this command once during start-up, and once for each capture operation as part of the setup sequence. It is also issued when clicking the Stop button to cancel a capture in progress.

Command

Variable length of 3 words (6 bytes) plus length × 4 words (8 bytes).

ID	Address	Length	Data	...
0007	aaaa	nnnn	dddd-dddd-dddd-dddd	...

The two argument words are the start address and the length of the slice to write, in little endian byte order. Both the address and the length refer to quantities of 64 bit (4 words or 8 bytes). Thus, if length is 10 the payload should consist of 40 words or 80 bytes.

The vendor software always writes a slice of length 10 beginning at address 0, thus completely resetting both the capture configuration as well as the capture status. The layout of the configuration and status memory is outlined in the description of command 0008.

Command 0008: Capture Status

This command reads the current capture configuration and status. Essentially, this command appears to read a slice from an internal memory block with 64-bit granularity -- the same memory space command 7 writes to. During idle periods, the software polls the channel state about 34 times per second for its live port status display. During a capture operation, it is necessary to poll the status in order to find out whether the capture buffer has been filled completely and samples can be retrieved.

Command

Fixed length of 3 words (6 bytes).

ID	Address	Length
0008	aaaa	nnnn

The two argument words are the start address and the length of the slice to read, in little endian byte order. Both the address and the length refer to quantities of 64 bit (4 words or 8 bytes). Thus, if length is 10 the reply will consist of 40 words or 80 bytes.

Although the vendor software always reads the full 10 fields of configuration and status information, it does not seem to be an actual requirement. Restricting reads to fields 5 to 9 in the sigrok driver works fine so far without any problems.

Response

The table below shows the response to the command as issued by the vendor software, i.e. with start address 0 and length 10. Each row is a 64-bit word in very much mixed up (6-5-8-7-2-1-4-3) byte order. Roughly, the first half is used to set up a capture operation, whereas the second half contains status information. Field 5 is an exception, as it is used for both setup and status.

Index	Byte offset	Value
0	00	Channel enable mask
1	08	Clock divider count
2	10	Trigger level mask
3	18	Trigger edge mask
4	20	Trigger enable mask
5	28	Capture memory fill level
6	30	Not used?
7	38	Running capture duration
8	40	Channel input state
9	48	Capture status flags

The channel enable mask and the input state are bit vectors, with bit 0 of the 64-bit word (after unmixing the byte order!) corresponding to CH1 and bit 33 corresponding to CH34. The enable mask shows which channels have been enabled for capturing. The bits in the input state vector signify whether the voltage at the corresponding input channels is currently low (0) or high (1).

Field 1 sets the max value of the counter which divides the internal clock to yield the sampling clock. This applies to all sample rates of 100MHz or less (125MHz is a special case: see control register 1094). The counter max value is calculated as follows: maxcount = 1 / (samplefreq * 10ns) - 1

Field 4 is the trigger enable mask. 1 enables triggering on a channel, 0 disables it. CH1 is mapped to bit 0 and CH34 to bit 33. Bits 34 and 35 are special and configure for external triggering. Bit 34 enables external triggering on the falling edge, bit 35 on the rising edge.

If a trigger channel is enabled, the corresponding bit in field 3 configures whether to trigger on level (0) or edge (1). The corresponding bit in field 2 then selects whether to trigger on low/falling (0) or high/rising (1).

During setup, field 5 is used to limit the (compressed) size of the captured data. During a running capture operation, field 5 indicates the current memory fill level.

Field 7 is the time passed since the first sample. For samplerates up to 100 MHz, the value is a duration in milliseconds. The 125 MHz setting is again special and uses the same timebase as the 100 MHz setting, i.e. field 7 can then be interpreted as the sample count × 10⁻⁵.

Field 8 records a snapshot of the signal level of all 34 channels at the time the status is being read.

Field 6 appears to be unused.

Capture Status Flags

The status flag bits are used as follows:

Bit 0	Bit 1	Bit 2	Bit 3	Bit 4	Bit 5
???	Capturing	???	???	Triggered	Memory available

The higher bits of field 9 appear to contain garbage (apparently copies of the bits from field 8).

Task Recipes

This section lists the commands issued by the software to perform a particular task.

Long Register Read

This sequence reads from a 64-bit wide internal memory, probably the same as that accessed by commands 7 and 8.

1. Write index to address register 0x10B4
2. Read dummy value from strobe register 0x10B0
3. Read high word from register 0x10BC
4. Read low word from register 0x10B8

Steps 3 and 4 appear to be interchangeable.

Long Register Write

This sequence writes to a 64-bit wide internal memory, probably the same as that accessed by commands 7 and 8.

1. Write index to address register 0x10B4
2. Write low word to register 0x10B8
3. Write high word to register 0x10BC
4. Write 0 (dummy value) to strobe register 0x10B0

Steps 3 and 4 appear to be interchangeable.

Initialization

1. Acquire control of USB device and select configuration 1
2. Send FPGA bitstream (default: internal clock) to EP 4 via bulk transfer
3. Device test sequence:
   1. Read long register 100; ignore result
   2. Read long register 100; result should be 0x1234567887654321
4. Capture setup/state test (vendor software does this, not mandatory):
   1. Write sequence 0..9 to capture setup fields (via command 7)
   2. Read back capture state (via command 8): 0..4 should be read back as is, 5..9 are trashed anyway
5. Memory test (vendor software does this, not mandatory):
   1. Write 2 to register 0x1074
   2. Write 0 to register 0x107C
   3. Read memory beginning at address 0 in chunks of 120 36-bit words, up to but not including 0x013FB0
   4. Read memory beginning at address 0x013FB0 in chunks of 8 36-bit words, up to but not including 0x014000

It does not seem to be necessary to use exactly the same read chunk length as the original vendor software. See the description of command 6 for constraints.

Poll channel state

The vendor software continuously polls the channel state even when idle. However, it is not mandatory to do so.

1. Poll frequency of signal at CH1 to CH4:
   1. Read register 0x10C0: value is frequency of CH1 signal
   2. Read register 0x10C4: value is frequency of CH2 signal
   3. Read register 0x10C8: value is frequency of CH3 signal
   4. Read register 0x10CC: value is frequency of CH4 signal
2. Poll signal level of all channels:
   1. Read capture state (via command 8): the signal level of all channels is recorded in field 8

Clocking Mode Switch

1. Transfer one of three FPGA bitstreams to EP 4:
   1. Configuration for internal clock
   2. Configuration for external clock, rising edge
   3. Configuration for external clock, falling edge

Signal Capture

1. Write 2 to register 0x1074
2. Write 1 to register 0x1074
3. Write 0x74 to long register 10
4. Write divider bypass flag to register 0x1094 (see description of register)
5. Write capture setup (command 7, address 0, length 10: see description of command)
6. Write 1 to long register 10
7. Wait for capture to finish:
   1. Poll capture state (command 8, address 0, length 10: see description of command)
   2. Report progress information (got trigger, cycles elapsed, memory fill percentage) to user
   3. Capture has finished once the memory available flag is reset

Cancel Signal Capture

1. Write 0 to long register 10
2. Write 0 to register 0x1094 (divider bypass flag)

After that, the memory available flag in the capture state should have been cleared. Continue in the same manner as for a regularly finished capture.

Read Captured Data

1. Read register 0x1078: value is the number of 36-bit words in the capture buffer
2. Write 1 to register 0x1094 (divider bypass flag)
3. Write 2 to register 0x1074
4. Write 4 to register 0x107C
5. Read capture buffer:
   1. Read memory beginning at address 4 in chunks of 120 36-bit words, up to but not including 0x03FFC4
   2. Read memory beginning at address 0x03FFC4 in chunks of 8 36-bit words, up to but not including 0x03FFF4
6. Write 0 to register 0x1094 (divider bypass flag)
7. Issue mystery command 5 with mystery data, but for some mystical reason only for the first three captures after application start

It does not seem to be necessary to use exactly the same read chunk length as the original vendor software. See the description of command 6 for constraints.

The sigrok driver works fine without ever issuing mystery command 5.

Shutdown

1. Transfer FPGA configuration for device shutdown to EP 4