BoSL Board v0.5

2023 September 28

The BoSL board rev 0.5.0 arrived and its time to test how it works. Lets jump into it!

Straight out of the packet:

• Plugging in a battery the idle current 840 µA.
• Touching a screw driver to the F1 TEST pad lights up the test LED.
• When the WDT_EN jumper is closed the current jumps up to ~10 mA. This seems appropriate, if it enables the voltage on VPP.

From now on WDT_EN will be closed

• Voltage is good on 3V3 and VBAT. Measured (3.370V and 3.941V respectively).
• The bootloader could be programmed using the normal process of Arduino as ISP. Specifically I used the [[1]] board library and selected the ATmega640 board. I then used BOD: 2.7V, bootloader: yes UART0, clock: external 8MHz, EEPROM: EEPROM retain, compiler LTO: LTO disabled, pinout: AVR pinout.
• Idle current now 18 mA.
• I then plugged in the board via USB and it was recognised as a serial port.
• Programs could be uploaded via USB now, the serial monitor analogue read, and the RX,TX, LED, and USB, LEDs work.

• I could get a response form the SIM7000 when I made the following changes to the BoSLpass script.
  • Replaced simCom soft serial object with Serial1
  • set PWRKEY 38, DTR 44, and defined SIM_BUF_EN 25
  • Pulled SIM_BUF_EN HIGH to turn on the voltage level converter.

• When I did this I could talk to the sim and recognise the simcard, but not register to the network. I suspect this is due to no Telstra reception here:

23:14:41.086 -> sending: AT+COPS=?
23:14:41.130 -> 
23:37:12.814 -> 
23:37:12.814 -> +COPS: (1,"vodafone AU","voda AU","50503",9),(1,"Optus AU","Optus","50502",9),,(0,1,2,3,4)

Firmware updates:

1. Now that we have the more pins it would we wise to wrap each call which writes to the serial of the SIM7000 to first check CTS and RTS and see that we are allowed to send.

Errors:

1. WDT_EN is not the correct name for the jumper as the WDT is disabled when closed. It should be not WDT_EN
2. Add silkscreen for female pin headers
3. Fix the pinout for U8, the MCP1700T-3302E which currently has the VPP and 3V3 swapped.

23 November 26

Today I am testing the idle current draw of the BoSL board. First is to test the idle current when Q1 shuts off +BATT from VPP. All tests here are with WDT_EN open.

I swapped the orientation of U8 (MCP1700T-3302E) as it was incorrect. Doing this resulted in the idle current from 1.1 mA to 0.87 mA.

I do not notice any "floating rails" however if WDT_EN is enabled and then disabled it will take a long time for the VPP rail to return to 0V as the connected capacitors need time to discharge.

Removing the Zener diode for reverse current protection (D10) results in a current of 0.64 mA or a drop of approximately 200 μA.

The voltage measured on PB2 of the ATtiny4 (U7) is just indistinguishable from 0V, I do not think that the current leaking through the ATtiny to the ATmega via the WDT_KICK signal is a cause for the excessive current draw.

One the ATtiny (U7) was removed the idle current dropped to 2.7 μA, essentially zero. From this I conclude that the Zener diode D1 and the ATtiny U7 were using the majority of the current when WDT_EN is left open. Part of the reason the ATtiny was consuming ~600 μA of current is that it was not yet programmed. Programming the ATtiny will be a key next step in fixing the high power off current draw while still being able to use the ATtiny as a WDT.

Again with WDT_EN open VPP and 3V3 may appear floating however this is because there is residual charge in the capacitors which takes a long time to deplete.

WDT_EN closed sleep current

Here I will not investigate the smallest sleep current possible when WDT_EN is closed, that is when VPP and 3V3 have power.

Running a simple script which only has sleepMode(SLEEP_POWER_DOWN); sleep(); The total current draw of the board falls to 360 μA.