HDMI 4-Channel Oscilloscope Demo
cgracey
Posts: 14,155
in Propeller 2
I was really wanting to see how the scope modes would work to build a simple oscilloscope with trigger. The SCOPE smart pin modes put out 8-bit samples on every clock and perform triggering functions based on signal levels.
So, this little scope program uses the DDS/Goertzel circuit to generate simultaneous sine, triangle, sawtooth, and square wave patterns in an FM mode, so that the frequency wanders between 350KHz and 1MHz. These four DDS output pins can be wired to the scope inputs for stimulus by connecting the following pins together:
P48 and P52
P49 and P53
P50 and P54
P51 and P55
That will give the scope something to trigger off of and display. The scope inputs are:
P52 = channel 0, level-triggers for scope operation
P53 = channel 1
P54 = channel 2
P55 = channel 3
You must drive some signal into channel 0 to generate a trigger, in case you don't use the DDS outputs.
An HDMI monitor connects on P32..P39.
This scope shows four channel traces, sampled every 4th clock in parallel. They could be sampled on every clock, but not much changes in so short of a time. The traces have persistence, so old traces gently fade away as new ones are plotted. It's doing 120 triggers/plots per second, though the monitor refreshes at only 60Hz.
There is an MP4 at the end of this post.
Here is the code. It's 172 longs:
So, this little scope program uses the DDS/Goertzel circuit to generate simultaneous sine, triangle, sawtooth, and square wave patterns in an FM mode, so that the frequency wanders between 350KHz and 1MHz. These four DDS output pins can be wired to the scope inputs for stimulus by connecting the following pins together:
P48 and P52
P49 and P53
P50 and P54
P51 and P55
That will give the scope something to trigger off of and display. The scope inputs are:
P52 = channel 0, level-triggers for scope operation
P53 = channel 1
P54 = channel 2
P55 = channel 3
You must drive some signal into channel 0 to generate a trigger, in case you don't use the DDS outputs.
An HDMI monitor connects on P32..P39.
This scope shows four channel traces, sampled every 4th clock in parallel. They could be sampled on every clock, but not much changes in so short of a time. The traces have persistence, so old traces gently fade away as new ones are plotted. It's doing 120 triggers/plots per second, though the monitor refreshes at only 60Hz.
There is an MP4 at the end of this post.
Here is the code. It's 172 longs:
'*************************************************
'* Use DDS to demonstrate 4-Channel SCOPE mode *
'*************************************************
CON scp_base = 52 'must be a multiple of 4, 1st pin is level-triggered input
dds_base = 48 'must be a multiple of 4, connect these pins to scop_base pins
hdmi_base = 32 'must be a multiple of 8
trigger_level = $90 'scope trigger level
arm_level = $70 'scope arm level
scope_filter = 0 'scope filter: 0 = 68-tap, 1 = 45-tap, 2 = 28-tap
dds_freq = 700_000.0 'nominal DDS frequency (without FM'ing)
freq = 250_000_000.0 'system clock frequency must be 250 MHz for HDMI
buffer = $1000 'sample buffer (4 KB)
bitmap = buffer+$1000 'HDMI bitmap (300 KB)
DAT org
hubset ##%1_000001_0000011000_1111_10_00 'config PLL, 20MHz/2*25*1 = 250MHz
waitx ##20_000_000 / 200 'allow crystal+PLL 5ms to stabilize
hubset ##%1_000001_0000011000_1111_10_11 'switch to PLL
setq ##($7FFFF - @end_of_pgm)/4 'clear hub RAM
wrlong #0,##@end_of_pgm
coginit #2,##@pgm_dds 'launch DDS
coginit #1,##@pgm_scp 'launch Scope
coginit #0,##@pgm_hdmi 'launch HDMI
'*********
'* DDS *
'*********
DAT org
pgm_dds cogid x 'init DAC pins for this cog's DAC channels
setnib dacmode,x,#2
wrpin dacmode,#3<<6+dds_base
dirh #3<<6+dds_base
' Make sine/triangle/sawtooth/square patterns in LUT
mov z,#$1FF 'make 512-sample DDS table in LUT
.lut shl z,#32-9 'channel 0 = sine
qrotate #127,z
shr z,#32-9
getqy y
mov x,z 'channel 1 = triangle
testb x,#8 wc
if_c not x
xor x,#$80
setbyte y,x,#1
mov x,z 'channel 2 = sawtooth
shr x,#1
xor x,#$80
setbyte y,x,#2
setbyte y,#$81,#3 'channel 3 = square
if_c setbyte y,#$7F,#3
wrlut y,z 'write square:sawtooth:triangle:sine into LUT
djnf z,#.lut 'loop until 512 samples
' Output DDS patterns and slowly FM
xcont dds_d,dds_s 'issue perpetual DDS/Goertzel command
.loop qrotate ##3_000_000,x 'slowly FM the signals
getqy y
add y,xfreq
setxfrq y
add x,#100
jmp #.loop
' Data
dacmode long %0000_0000_000_10110_00000000_01_00000_0 'DAC mode, cog DAC channels
xfreq long round(dds_freq/freq * 65536.0 * 32768.0) 'streamer frequency value
dds_d long %1111_1111_0000_0111<<16 + $FFFF 'DDS/Goertzel mode, continuous
dds_s long %0000_0000_000_000000000 'DDS/Goertzel mode, no input
x res 1
y res 1
z res 1
'***********
'* Scope *
'***********
'
DAT org
pgm_scp wrpin .scpmode,#3<<6+scp_base 'init ADC/scope pins
wxpin .scp_x,#3<<6+scp_base
dirh #3<<6+scp_base
setscp #1<<6 + scp_base 'enable scope channels
setse1 #%001<<6 + scp_base 'set SE1 event to first scope pin trigger
setxfrq ##$2000_0000 'set streamer NCO frequency to sample every 4th clock
' Capture waveforms before and after trigger
.loop wrfast #$1000/64,##buffer 'set wrfast to wrap 1k-sample circular buffer of 4 scope channels
xinit .scp_d,#0 'issue 4-ADC8 capture command
waitx ##320*4 'allow time to fill first half of buffer
akpin #scp_base 'acknowledge trigger pin to clear
pollse1 'clear any old trigger event
akpin #scp_base 'acknowledge trigger pin to clear
waitse1 'wait for new trigger event
getptr .x 'capture write pointer, reflects trigger point
waitx ##320*4 'allow time to fill second half of buffer
xstop 'stop streamer, ~640 samples gathered within 1k sample buffer
sub .x,##320*4 'determine start of buffer so that trigger point is in the middle
and .x,##$0FFC
or .x,##buffer
rdfast #$1000/64,##buffer 'set rdfast to wrap 1k-sample buffer
.scan getptr .y 'advance to starting sample (320 samples before trigger)
cmp .y,.x wz
if_nz rflong .p
if_nz jmp #.scan
' Dim existing pixels in bitmap
loc ptra,#bitmap
mov .y,#480
.dimline setq2 #640/4-1 'read in 640 pixels
rdlong 0,ptra
mov ptrb,#0
rep @.r,#640/4 'dim 640 rgbi8 pixels
rdlut .p,ptrb
mov .q,.p
and .q,.rgbmasks
not .p
or .p,.rgbmasks
addpix .p,.incbytes
not .p
or .p,.q
wrlut .p,ptrb++
.r
setq2 #640/4-1 'write back 640 pixels
wrlong 0,ptra++
djnz .y,#.dimline
' Plot waveforms
mov .x,#0
.xloop rflong .p 'get 4-channel sample
mov .q,#480 'plot channel 3 first
rep @.plot,#4 'ready to plot 4 channels
getbyte .y,.p,#3 'get sample
shr .y,#1 'divide by 2 to fit screen
subr .y,.q 'flip and vertically position
mul .y,.xsize 'get bitmap pixel address
add .y,.x
add .y,.bitmap
wrbyte .color,.y 'plot pixel
shl .p,#8 'ready next sample
sub .q,#117 'ready next vertical position
ror .color,#8 'ready next color
.plot
incmod .x,.xlimit wc 'loop until all pixels plotted
if_nc jmp #.xloop
' Plot trigger level marker
mov .y,.scp_x 'get trigger level
shr .y,#9
subr .y,#128
mul .y,.xsize
add .y,#320-9
add .y,.bitmap
setq #4-1
wrlong ##$FFFFFFFF,.y 'white marker
' Toggle P56 for a speed indicator, then loop
drvnot #56
jmp #.loop 'loop
' Data
.scpmode long %0000_0000_000_100011_0000000_00_11010_0 'ADC/scope mode
.scp_d long %1111_0000_1000_0110<<16 + $FFFF 'DDS/Goertzel mode
.scp_x long (trigger_level & $FC)<<8 + (arm_level & $FC) + scope_filter
.rgbmasks long $E0E0E0E0
.incbytes long $01010101
.xlimit long 640-1
.xsize long 640
.bitmap long bitmap
.color long $5F_BF_DF_1F 'initial trace colors
.x res 1
.y res 1
.p res 1
.q res 1
'*********************************
'* HDMI 640 x 480 x 8bpp rgbi8 *
'*********************************
DAT org
pgm_hdmi setcmod #$100 'enable HDMI mode
drvl #7<<6 + hdmi_base 'enable HDMI pins
wrpin ##%111001<<8,#7<<6 + hdmi_base 'set 1.5k low drive on HDMI pins
setxfrq ##$0CCCCCCC+1 'set streamer freq to 1/10th clk (25 MHz)
rdfast ##640*480/64,##bitmap 'set rdfast to wrap on 300KB bitmap
' Field loop
.field mov .hsync0,.sync_000 'vsync off
mov .hsync1,.sync_001
callpa #10,#.blank 'top blanks
mov .i,#480 'set visible lines
.line call #.hsync 'do horizontal sync
xcont .m_rf,#0 'do visible line
djnz .i,#.line 'another line?
callpa #33,#.blank 'bottom blanks
mov .hsync0,.sync_222 'vsync on
mov .hsync1,.sync_223
callpa #2,#.blank 'vertical sync blanks
jmp #.field 'loop
' Subroutines
.blank call #.hsync 'blank lines
xcont .m_vi,.hsync0
_ret_ djnz pa,#.blank
.hsync xcont .m_bs,.hsync0 'horizontal sync
xzero .m_sn,.hsync1
_ret_ xcont .m_bv,.hsync0
' Data
'.sync_000 long %1101010100_1101010100_1101010100_10 '
'.sync_001 long %1101010100_1101010100_0010101011_10 ' hsync
'.sync_222 long %0101010100_0101010100_0101010100_10 'vsync
'.sync_223 long %0101010100_0101010100_1010101011_10 'vsync + hsync
.sync_000 long %1101010100_1101010100_1101010100_10 '
.sync_001 long %1101010100_1101010100_0010101011_10 ' hsync
.sync_222 long %1101010100_1101010100_0101010100_10 'vsync
.sync_223 long %1101010100_1101010100_1010101011_10 'vsync + hsync
.m_bs long $70810000 + hdmi_base<<17 + 16 'before sync
.m_sn long $70810000 + hdmi_base<<17 + 96 'sync
.m_bv long $70810000 + hdmi_base<<17 + 48 'before visible
.m_vi long $70810000 + hdmi_base<<17 + 640 'visible
.m_rf long $B0830000 + hdmi_base<<17 + 640 'visible rfbyte luma8
.hsync0 res 1
.hsync1 res 1
.i res 1
end_of_pgm
1024 x 576 - 161K
Comments
I've jumpered P48 to P52 to get the trigger operating, it is using 364 mA at 1.83 Volts on VDD.
EDIT:
Low step is because mp4 is upside down and the step is actually a high point.
What is the bandwidth/rise time of the Analog here ?
1080p could show all those channels at full scale, but the P2 would need 2MB of hub RAM. Shy of that, we'd have to do some coding to get around needing so much RAM. We could not do persistence and get by with much less, at maybe two bits per pixel.
Yes, that little ripple reveals that adjacent I/O pins capacitively couple into their neighbors' high-impedance ADC nodes. Next time we make a layout change, we need to put a ground cage around the ADC innards.
In the case of the sawtooth and square wave signals, it's a full 3.3 volt step in about 3ns.
Ken Gracey
Kind regards, Samuel Lourenço
Thanks. I was always wondering how exactly to get the trigger into the center of the display. I realized that you need to gather at least half a screen of information, wait for a trigger, record the address it was writing to, and then load let it load another half screen of information. Then, your trigger is right in the middle of the screen every time.
That's crosstalk, definitely, and its outside the chip. This could be solved using controlled impedance and trace separation using ground planes. This looks like the effect of capacitive coupling between two adjacent traces on the board.
Kind regards, Samuel Lourenço
If you space the ADC pins more, does that issue reduce ?
I'll have to put in something different now into my demo. I had started a somewhat similar thing with a frequency shifting sine wave on the screen yesterday and was going to add some AM to it, but it was not taking any real pin data, purely synthetic. You went next level and used real IO data! Awesome.
Having the built in scope mode is going to be very handy when working with real waveforms.
Your picture looks 10x better than mine. Yes, it does seem to be external, based on its duration. I think we do have small ground traces between signal traces on the inner layers. They are also sandwiched with power and ground. What I will do to determine if this is, indeed the case, is make one of the DACs 990 ohms, while leaving the others at 123 ohms. I will see if it gets pushed around even further. I am pretty sure you are right, that it is outside of the chip. Much better than having cross-talk problems inside the chip.
Yes there were thin ground guard traces between the P2 signal traces last time I looked at the gerbers
I suspect that is related to what i'm experiencing with the hyperram timing too.
The problem is that a common-centroid resistor array brings in all four signals before they go to selector switches which pick just one. By the time one is picked, it has noise in it coupled from the other three signals.
This is going to be a problem which is not entirely solvable. If you want to measure just one dynamic signal precisely, you could bring it into two pins, an even/odd pair, and then not worry about crosstalk. Or, make sure the even/odd companion pin isn't changing during measurements. You could not, however, get rid of coupling between the GIO and VIO input channels and the incoming signal from the pin. So, GIO/VIO calibration is only reasonable if you have near-DC signals that are not very dynamic.
To solve this, the resistor array needs to be redesigned to separate and not interleave resistor segments of different signals. Or, better, the switches need to be put BEFORE the resistor array, so nothing bad ever gets into the array, in the first place.
This will take a layout change to fully remedy.
I will post some pictures of the degree of crosstalk in the scope demo. I wish I would have simulated for this kind of problem. It would have been easily solved if I had known about it. It should have occurred to me.
Hindsight is wonderful. But you have done a splendid job in getting the P2 done!!!
These things can be solved on the next variant (eg 4 cogs). Meanwhile, we will live with current issues, whatever they may be. There are no show-stoppers here.
Here are two square waves incoming on an even/odd pin pair, coupling transitions onto eachother's flat spots:
These are all worst-case coupling scenarios.
For audio-frequency signals, these couplings may be barely audible, but for high-speed signals, they are visually apparent.
I put square wave at the top,
then sine wave,
then sawtooth,
then ramp
I'm not convinced you can't pass the data through some kind of deconvolution to recover the original signals. What you're seeing is a matrix a bit like
[ 0.9 0.1 0.0 0.0
0.1 0.8 0.1 0.0
0.0 0.1 0.8 0.1
0.0 0.0 0.1 0.9 ] which gets multiplied by the 4 signals [A B C D]' to produce what we see. Only the coefficients may be frequency dependent if the effects are capacitive. If this is the case we could find the inverse matrix (model) that represents the coupling and apply that to approximate the original signals, at least improve them
It struck me today just how impressive the scope mode actually is. If you have a 1 MHz input signal and want to plot 10 cycles of that on a 640 px wide display, you want about 60 samples per 1 MHz cycle, ie a tradition SAR ADC converter would want to sample at 60 Msps to achieve this. And here things are happening simultaneously on 4 channels. Its a great demo despite the crosstalk.
We need to calculate the differences between adjacent samples for each channel, and then subtract those differences from each other channel.
PinA_delta = PinA_new - PinA_old
PinB_delta = PinB_new - PinB_old
PinA_final = PinA_new - PinB_delta * scale
PinB_final = PinB_new - PinA_delta * scale
I am going to try this now...
Here it is uncompensated, where the adjacent pin input attacks the primary pin input within an odd/even pin pair:
Here it is with compensation applied. You can see that while it nulls out long transition effects, it needs to be offset by a sample or two, in order to register in time properly:
This is good enough for a window-filtered oscilloscope display which is limited to 8 bits.
Does it hold as you go up or down in frequency?
The chip is running at 250MHz and the SCOPE channels are being captured every 2nd clock. So, a two-sample delay is a 4-clock delay, which at 250MHz is a 16ns delay. The deltas are computed from -20ns to -16ns relative to the sample at 0ns. The scale factor applied to the delta is 200/256 (0.78125).
This is sampled on every 4th clock. The deltas are computed from the 2nd and 3rd samples behind the current. Compensation scale factor is 100/256.