I'm working on a project that uses the Fanstel BC840m module. The project uses a custom PCB design, and everything was going smoothly until... I enabled DCDC1, stopped building in DEBUG, and switched to battery.
Issue
When I enable use of DCDC1 I experience frequent (~5% probability) resets after BLE advertisements.
Hardware
- Custom PCB
- XTAL populated and configured
- Fanstel BC840m (52840 based)
- Docs state the module has components to enable DCDC1
- Coin-cell battery (CR2032)
Software
- SDK v16
- S140 v7.0.1
Observations
- Reset occurs immediately after completion of BLE advertisements
- Verified by oscilloscope on VDD
- Reset occurs, with less frequency, when powered by Nordic PPK and voltage reduced to ~2.5-2.5V
- I have many custom PCB units. Reset frequency varies by unit. Some are very bad (seconds), some are better (hours)
Troubleshooting steps
- First instinct is voltage drop problem, so I added additional capacitance to the host PCB, and moved that closer to the Fanstel module connections. This might have made a slight improvement, but not much if any.
- I've attached an oscilloscope to VDD to watch for drops. 2 snaps are attached.
- I've read nearly every post here that mentions DCDC. I've learned a lot, and I know that coin-cell batteries as a power supply are difficult... But nothing I've seen seems to explain this.
Screenshots
Here are two screenshots watching VDD during a BLE advertisement TX cycle. Note: I have masked off 2 advertising channels, so there is only 1 BLE transmission per cycle. The first screenshot here is of a "clean" TX where the unit did not reset. I labeled several key events with letters A, B, C, D. Can someone tell me what is causing D? D is exactly where the reset occurs, you can compare the two screenshots to see. Sorry they're different time-scales so they don't line up perfectly.
A - Some sort of soft wakeup, I'm guessing to get the CPU and radio ready for TX
B - DCDC power-up and charge
C - BLE TX
D - ? Why the dip? Is this where it's switching back to LDO? This is the point at which resets occur. See next picture.
FWIW -- I also have a custom PCB that uses the Fanstel BT840 module (still 52840 based). Under the exact same software configuration (and nearly equivalent schematic) I have not experienced a single reset.
So does anyone have ideas? What is happening at D? Is it something I can control to prevent the reset? Any ideas on what the underlying cause may be? I would love to find a software work-around, even if there is some penalty, but I'd also like to identify the true cause and resolve that for the next HW spin.
