PCAP - Position Capture

Position capture has the capability to capture anything that is happening on the pos_bus or bit_bus. It listens to ENABLE, GATE and CAPTURE signals, and can capture the value at capture, sum, min and max.

Fields

Name	Type	Description
ENABLE	bit_mux	After arm, when high start capture, when low disarm
GATE	bit_mux	After enable, only process gated values if high
TRIG	bit_mux	On selected edge capture current value and gated data
TRIG_EDGE	param enum	Which edge of capture input signal triggers capture 0 Rising 1 Falling 2 Either
SHIFT_SUM	param uint 8	Shift sum/samples data, use if > 2**32 samples required in sum/average
ACTIVE	bit_out	Data capture in progress
TS_START	ext_out timestamp	Timestamp of first gate high in current capture relative to enable
TS_END	ext_out timestamp	Timestamp of last gate high +1 in current capture relative to enable
TS_TRIG	ext_out timestamp	Timestamp of capture event relative to enable
SAMPLES	ext_out samples	Number of gated samples in the current capture
BITS0	ext_out bits 0	Quadrant 0 of bit_bus
BITS1	ext_out bits 1	Quadrant 1 of bit_bus
BITS2	ext_out bits 2	Quadrant 2 of bit_bus
BITS3	ext_out bits 3	Quadrant 3 of bit_bus
HEALTH	read enum	Was last capture successful? 0 OK 1 Capture events too close together 2 Samples overflow

Arming

To start off the block an arm signal is required with a write to *PCAP.ARM=. The active signal is raised immediately on ARM, and dropped either on *PCAP.DISARM:

(Source code, png, hires.png, pdf)

Or on the falling edge of ENABLE:

(Source code, png, hires.png, pdf)

Capturing fields

Capturing fields is done by specifying a series of WRITE addresses. These are made up of a mode in the bottom 4 bits, and an index in the 6 bits above them. Indexes < 32 refer to entries on the pos_bus, while indexes >= 32 are extra entries specific to PCAP, like timestamps and number of gated samples. The values sent via the WRITE register are written from the TCP server, so will not be visible to end users.

Data is ticked out one at a time from the DATA attribute, then sent to the TCP server over DMA, before being sent to the user. It is reconstructed into a table in each of the examples below for ease of reading.

The following example shows PCAP being configured to capture the timestamp when CAPTURE goes high (0x24 is the bottom 32-bits of TS_CAPTURE).

(Source code, png, hires.png, pdf)

Row	0x240
0	2
1	6

Pos bus capture

As well as general fields like the timestamp, any pos_bus index can be captured. Pos bus fields have multiple modes that they can capture in.

Mode 0 - Value

This gives an instantaneous capture of value no matter what the state of GATE:

(Source code, png, hires.png, pdf)

Row	0x50
0	20
1	100
2	6

Mode 1 - Difference

This is mainly used for something like an incrementing counter value. It will only count the differences while GATE was high:

(Source code, png, hires.png, pdf)

Row	0xB1
0	10
1	-5

Mode 2/3 - Sum Lo/Hi

Mode 2 is the lower 32-bits of the sum of all samples while GATE was high:

(Source code, png, hires.png, pdf)

Row	0x32
0	6
1	21
2	206

Mode 2 and 3 together gives the full 64-bits of sum, needed for any sizeable values on the pos_bus:

(Source code, png, hires.png, pdf)

Row	0x22	0x23
0	1073741824	0
1	-1073741824	0
2	-2147483648	2
3	-1073741824	-1
4	-1073741824	-2

If long frame times (> 2**32 SAMPLES, > 30s), are to be used, then SHIFT_SUM can be used to shift both the sum and SAMPLES field by up to 8-bits to accomodate up to 125 hour frames. This example demonstrates the effect with smaller numbers:

(Source code, png, hires.png, pdf)

Row	0x92	0x260
0	40	1
1	36	1
2	-13	1
3	0	0

Mode 4/5 - Min/Max

Both of these modes calculate statistics on the value while GATE is high.

Mode 4 produces the min of all values or zero if the gate was low for all of the current capture:

(Source code, png, hires.png, pdf)

Row	0x84
0	10
1	20
2	21
3	2147483647

Mode 5 produces the max of all values in a similar way:

(Source code, png, hires.png, pdf)

Row	0x45
0	20
1	20
2	22
3	-2147483648

Number of samples

There is a SAMPLES field that can be captured that will give the number of clock ticks that GATE was high during a single CAPTURE. This field allows the TCP server to offer “Mean” as a capture option, dividing “Sum” by SAMPLES to get the mean value of the field during the capture period. It can also be captured separately to give the gate length:

(Source code, png, hires.png, pdf)

Row	0x260
0	4
1	3
2	2
3	0

Timestamps

As well as the timestamp of the capture signal, timestamps can also be generated for the start of each capture period (first gate high signal) and end (the tick after the last gate high). These are again split into two 32-bit segments so only the lower bits need to be captured for short captures. In the following example we capture TS_START (0x20), TS_END (0x22) and TS_CAPTURE (0x24) lower bits:

(Source code, png, hires.png, pdf)

Row	0x200	0x220	0x240
0	0	4	4
1	4	8	9
2	11	13	13
3	-1	-1	16

Bit bus capture

The state of the bit bus at capture can also be captured. It is split into 4 quadrants of 32-bits each. For example, to capture signals 0..31 on the bit bus we would use BITS0 (0x27):

(Source code, png, hires.png, pdf)

Row	0x270
0	0
1	4
2	20
3	16

By capturing all 4 quadrants (0x27..0x2A) we get the whole bit bus:

(Source code, png, hires.png, pdf)

Row	0x270	0x280	0x290	0x2A0
0	4	0	0	0
1	4	67108864	0	0
2	4	67108864	0	32
3	1028	67108864	0	32

Triggering options

ENABLE and GATE are level triggered, with ENABLE used for marking the start and end of the entire acquisition, and GATE used to accept or reject samples within a single capture from the acquisition. CAPTURE is edge triggered with an option to trigger on rising, falling or both edges.

Triggering on rising is the default, explored in the preceding examples. Triggering on falling edge would be used if you have a gate signal that marks the capture boundaries and want sum or difference data within. For example, to capture the amount POS[1] changes in each capture gate we could connect GATE and CAPTURE to the same signal:

(Source code, png, hires.png, pdf)

Row	0x11
0	10
1	-9

Another option would be a gap-less acquisition of sum while gate is high with capture boundaries marked with a toggle of CAPTURE:

(Source code, png, hires.png, pdf)

Row	0x12
0	30
1	178
2	39

Error conditions

The distance between capture signals must be at least the number of 32-bit capture fields. If 2 capture signals are too close together HEALTH will be set to 1 (Capture events too close together).

In this example there are 3 fields captured (TS_CAPTURE_L, TS_CAPTURE_H, SAMPLES), but only 2 clock ticks between the 2nd and 3rd capture signals:

(Source code, png, hires.png, pdf)

Row	0x240	0x250	0x260
0	1	0	0
1	5	0	0