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by James Adams
Fri Jul 02, 2021 12:51 pm
Forum: General
Topic: JLCPCB and RP2040 (was Choosing a crystal for the RP2040)
Replies: 6
Views: 607

Re: JLCPCB and RP2040 (was Choosing a crystal for the RP2040)

ESR is important (it should be max 50R) and allowed crystal drive level (as long as it can tolerate max 200uW you should be fine). Otherwise you risk overdriving your crystal (which can cause it to fail).

Load caps can vary a bit, as long as you match them to the crystal requirement.
by James Adams
Tue Mar 09, 2021 11:51 am
Forum: General
Topic: ADC Differential Non-Linearity
Replies: 71
Views: 6282

Re: ADC Differential Non-Linearity

A quick update on this - the issues are understood (we wanted to make sure all the data people are seeing/presenting was consistent with the known issues as well as make sure we completely understood the root cause(s) in the ADC design). We are busy working on getting comprehensive characterisation ...
by James Adams
Tue Feb 09, 2021 5:10 pm
Forum: General
Topic: ADC Differential Non-Linearity
Replies: 71
Views: 6282

Re: ADC Differential Non-Linearity

We are aware of the quirks with the ADC - the intent is to add something to the datasheet once we've fully understood the cause(s) and gathered some more qualification data (we're working with the IP vendor).
by James Adams
Thu Jan 28, 2021 10:38 pm
Forum: General
Topic: Pico external power source docs - NOT wrong way round!
Replies: 14
Views: 1581

Re: Pico external power source docs - NOT wrong way round!

"OK, neat idea, ish. I'll have a play with this config soon to add it to the toolkit. But this -> "which is actually only strictly true if the secondary source+PFET body diode drop is lower than VBUS" - which has some real possibility of being very strictly not true given the use of 1...
by James Adams
Wed Jan 27, 2021 9:41 pm
Forum: General
Topic: Pico external power source docs - NOT wrong way round!
Replies: 14
Views: 1581

Re: Pico external power source docs - wrong way round?

The PFET is the correct way around (for this application). The body diode behaves like a diode, so similar to the two-schottky-diode approach, except when VBUS is not present, or below Vt, the FET turns on fully, removing the diode drop (ta-da). Possibly you are being confused by the sentence "...
by James Adams
Thu Aug 27, 2020 4:44 pm
Forum: Compute Module
Topic: CM RUN Pin 177 - min pulse width?
Replies: 4
Views: 935

Re: CM RUN Pin 177 - min pulse width?

A 1us pulse should be more than enough.
by James Adams
Tue Jan 14, 2020 11:51 am
Forum: Advanced users
Topic: Missing labels on the schematics 4b
Replies: 14
Views: 2966

Re: Missing labels on the schematics 4b

R13: 5V bleeder to pull 5V down in sensible time when unplugged. WB_VBAT: WiFi/BT supply, needs to be >3.6V for RF PA LDO to be in regulation. "Strangely, the LED is turned off by shorting it out - kind of weird" - not really, it guarantees the LED is ON until actively driven OFF (which is...
by James Adams
Mon Aug 05, 2019 7:46 am
Forum: Troubleshooting
Topic: Raspberry Pi 4 - Dead USB, no VBUS
Replies: 12
Views: 2355

Re: Raspberry Pi 4 - Dead USB, no VBUS

The FET holds off the switch enable until the system reset is released (as the VLI USBHPE output is somewhat badly behaved at power up) - see attached schematic fragment.
VLI_VBUS_EN.png
VLI_VBUS_EN.png (93.47 KiB) Viewed 2049 times
by James Adams
Wed Jul 10, 2019 9:59 am
Forum: Troubleshooting
Topic: Pi 4 fails to boot when active USB switch attached
Replies: 23
Views: 10327

Re: Pi 4 fails to boot when active USB switch attached

USB peripherals should never back-power VBUS like this - so this is bad peripheral behaviour. VBUS applied to an unpowered Pi will cause the rails to charge up and the PMIC will then not start (as it is designed to only start from a known state - all rails less than ~200mV - so it makes sure the pow...
by James Adams
Wed Jul 10, 2019 9:21 am
Forum: General discussion
Topic: Micro/type-c USB power supplies immunity to backpowering validity/proof
Replies: 13
Views: 1465

Re: Micro/type-c USB power supplies immunity to backpowering validity/proof

The structure of flyback AC-DC PSUs (wall-wart type) means they generally don't (can't) sink current. As long as your HAT does not sink current and will stop sourcing current when its output voltage is at the regulated set point (5V) or higher it should not 'fight' with any accidentally plugged in U...
by James Adams
Tue Jun 25, 2019 10:24 am
Forum: Advanced users
Topic: Missing labels on the schematics 4b
Replies: 14
Views: 2966

Re: Missing labels on the schematics 4b

These have been removed as they aren't generally useful - they aren't standard parts that are available in the channel and so you can't get hold of the parts or datasheets.

Also, we often change manufacturers (of connectors, say) so they can change over time.

Is there some specific info you need?
by James Adams
Tue Apr 30, 2019 8:01 am
Forum: Compute Module
Topic: Schematics for the CM3+
Replies: 2
Views: 871

Re: Schematics for the CM3+

Yes the CM3+ is the same as CM3 except using BCM2837B (package with the can) instead of BCM2837A.
by James Adams
Thu Sep 21, 2017 11:39 am
Forum: Compute Module
Topic: CMIO v3 design files
Replies: 6
Views: 3193

Re: CMIO v3 design files

by James Adams
Thu Sep 21, 2017 9:53 am
Forum: Compute Module
Topic: Flickering HDMI on custom CM3 board
Replies: 22
Views: 7111

Re: Flickering HDMI on custom CM3 board

This could also be a power supply noise issue - the HDMI PHY uses both 3V3 and 1V8, so one thing to check is your supplies are clean and sequenced properly (compare them with CMIO3 board).
by James Adams
Wed Sep 13, 2017 8:27 am
Forum: Compute Module
Topic: Boot problem with V3 on card designed past year
Replies: 22
Views: 6566

Re: Boot problem with V3 on card designed past year

When the CM3 does not start can you measure the voltage across C42 on the CM3 (bottom right corner of the module next to L1). I'm finding it hard to explain why it works on a CMIO3 but on a CMIO1 (with VBAT at 5V) it does not - as these boards really are identical except the VBAT power and the CMIO3...
by James Adams
Tue Sep 12, 2017 10:30 am
Forum: Compute Module
Topic: Boot problem with V3 on card designed past year
Replies: 22
Views: 6566

Re: Boot problem with V3 on card designed past year

2 things:

Can you try delaying RUN a bit more before bringing it high?

I'm still concerned about the residual voltage on the supplies - why are they starting at ~500mV?

James
by James Adams
Mon Sep 11, 2017 11:42 am
Forum: Compute Module
Topic: Boot problem with V3 on card designed past year
Replies: 22
Views: 6566

Re: Boot problem with V3 on card designed past year

Can you explain why there are 2 3V3 traces (blue and yellow)? Also can you explain why the traces start at ~500mV (the yellow one then drops down before rising again)? Residual voltages on power rails is a not-uncommon cause of these kind of power-up issues. Some idea of your powerchain architecture...
by James Adams
Mon Sep 11, 2017 8:23 am
Forum: Compute Module
Topic: Boot problem with V3 on card designed past year
Replies: 22
Views: 6566

Re: Boot problem with V3 on card designed past year

With your LED on EMMC_EN_N you are just measuring the forward LED voltage drop (through 10K to 3V3). Seems to be working to me. This feels like a power supply issue, the CM3 takes a lot more current from VBAT. What is the capability of your upstream power supply? (I.e. the 5V input) You need to look...
by James Adams
Mon Sep 04, 2017 2:43 pm
Forum: Compute Module
Topic: 3V3 voltage range
Replies: 1
Views: 1349

Re: 3V3 voltage range

Hi Milek7 while we don't guarantee anything outside of 3V3+/- 5% (mostly due to HDMI voltage requirements to meet HDMI certification) I would be surprised if you had any issues running down at 3.0V
by James Adams
Wed Aug 02, 2017 1:24 pm
Forum: Compute Module
Topic: Broken Compute Module
Replies: 5
Views: 2144

Re: Broken Compute Module

Hi themeerd. Are you using the very latest rpiboot software, and what platform are you using to flash with? (PC, Mac, Pi?, which OS?) - are you using a Compute Module IO board and if so which version? What you describe shouldn't be happening, though gsh has fixed a couple of bugs in rpiboot fairly r...
by James Adams
Wed Aug 02, 2017 10:32 am
Forum: Compute Module
Topic: Voltages needed for fast shutdown
Replies: 1
Views: 1125

Re: Voltages needed for fast shutdown

Hi JohW. The supply rails need to be up at all times unless you are powering the entire system down - the reason is there are parasitic diodes in the silicon device(s), as is common to most devices with multiple power domains, that can turn on if say the 3V3 domain goes away but the 1V8 stays up (1V...
by James Adams
Thu Jan 26, 2017 9:30 am
Forum: Compute Module
Topic: Is CM3 compatbile with the CM1 development board?
Replies: 23
Views: 28850

Re: Is CM3 compatbile with the CM1 development board?

Note that the CMIO-V1 supplies the Compute Module's VBAT from the 3V3, whereas on CMIO-V3 it is supplied from 5V. CM3 can pull significantly more current from VBAT, so if you run heavy CPU loads and/or have anything else using 3V3 it could be marginal. We recommend removing R1 and using a wire to co...
by James Adams
Fri Jan 20, 2017 8:57 am
Forum: Compute Module
Topic: CM3 Board Adapters?
Replies: 12
Views: 3708

Re: CM3 Board Adapters?

Hi Bruce where did you order the kit from, Farnell or RS?
by James Adams
Wed Oct 26, 2016 7:48 am
Forum: Compute Module
Topic: Compute Module Projects
Replies: 3
Views: 5823

Re: Compute Module Projects

Nice job!
by James Adams
Fri Aug 14, 2015 9:24 pm
Forum: Compute Module
Topic: No HDMI on custom board, but fine on OEM dev board.
Replies: 14
Views: 4006

Re: No HDMI on custom board, but fine on OEM dev board.

GPIO46 should be LOW (indicating HDMI hot plug) so that is likely your problem.

James

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