Date start: 2024-06-02
Date end: 2024-06-11
Operator: FBä

Overview

DUT

No.: 1
BMM version: 0.1.0

Modifications

• Nothing, yet

Notes

• Testing took place at room temperature (22..25 °C).

Power supply

The BMM is supplied by either V_Bus from USB or external 5V. The Pi Pico generates 3.3V and passes it on to the BMM board.

Setup for idle power measurements:

• Pico W running Micropython 1.23.0
• no interface active, no LED, no beeper

Pi Pico USB-V_Sys

Voltage drop across Pi Pico’s internal Schottky diode at idle current is ~0.2V.

Reverse polarity protection (idle, external supply)

DMP3099L V_GS(th,max): 2.1V (I_D: 250 µA)

Input voltage drop (measurement at idle power): V_F1 + V_DS(Q1) + V_FB1 = V_Sys -V_Ext

Notes:

• Reverse polarity protection is functional.
• V~Ext(min) should be 3.3V to make FET conduct properly (with some headroom). At this voltage, the body diode forward drop would also allow operation with V_Sys > 1.8 (with further increased losses).
• At V_Ext < 5V the RS-485 cannot supply external hardware at all (see ISOW1412 datasheet).

Current/power consumption (idle, external supply)

U6 (TCA9539) write, 400 kHz

• Functional (read, write, /int, /reset).
• Pushing buttons sets /int line (correct).
• Pulling reset low seems to be obligatory on start-up with the current “power sequencing”.

EEPROM

U3 (M24C08-DRDW)

U4 (CAV24M01YE-GT3)

Functional.

Beeper, LS1 (PKLCS1212E20A0-R1)

• Functional
• Voltage inverter/doubler to increase sound level works.

Digital IO

Notes:

• Tested at room temperature of 25°C

Inputs

Use V_In ≥ 3V as designed.

Input threshold

Notes:

• Work down to 1.69V. This is sufficient headroom for differing temperatures and ageing.
• Deviations due to different forward voltages will be quite big at lowest input voltages.
• All inputs were confirmed to have a very similar threshold (does not mean, all parts batches have the same characteristic).

Illustrative threshold voltage measurement:

Temperature rise

Input(s) driven at 26V for 15 minutes:

Outputs

Notes:

• Only current limited voltages may be connected
• I_C = 2.2mA is the guaranteed current over temperature/ageing.

Illustrative output resistance measurement:

Debug UART

Tested by echoing incoming data.

RS-485 transceiver, U17 (ISOW1412BDFMR)

Isolated output voltage V_Bus (idle):

Supply current increase at V_Sys = 5V (no bus connected):

Measurement of short test transmission:

Note: Receiving remains untested (no 2nd node available), but there’s no reason to believe it would not be functional.

isoSPI transceiver, U7 (LTC6820IMS)

Setup:

• Forward direction, only.
• Driving 50cm cable (of actually unknown impedance)
• 100Ω terminantion (no isoSPI device connected)

Pulse output measurement (isolated by transformer):

Results:

• Pulse durations are as expected.
• Signal level is lower (700mVp) than expected (20IF_IBIAS (R_Term/2) = 20833µA (100Ω/2) = 850mVp). But it should be sufficient with comparator threshold of 333mV and an environment with little noise. It also fits to BMFE values (which should be updated in the future as well).

Changes

Version 0.1.0 (retroactive)

• Add pull-ups for digital inputs.

TODO:

• LTC6820 Pin 3 1kΩ pull-up (open drain output)
• LTC6820 (u7+U8) Pin 13 (Slow) to GND (> 200 kHz)
• Replace R43 and R45 with 1.3kΩ, R44/R47 and R46/R48 with 1.3kΩ to increase signal level slightly and harmonize dimensioning with BMFE.

Version 0.1.1

• Add pull-ups for digital inputs.

TODO:

• LTC6820 Pin 3 1kΩ pull-up (open drain output)
• LTC6820 (u7+U8) Pin 13 (Slow) to GND (> 200 kHz)
• Replace R43 and R45 with 1.3kΩ, R44/R47 and R46/R48 with 1.3kΩ to increase signal level slightly and harmonize dimensioning with BMFE.