earckens
Member
In the schematic below I use a classical setup for a logic level shifter. However, in this case, when using this for an SPI bus MISO signal the mosfet refuses to switch its drain to low. Attached scope output: yellow is MISO low level, blue is MISO high level.
Board track lengths for this circuit are less than 1cm.
Vcc in my case is 3.3V so a level shifter is not technically required but the issue is that the high level MISO can change -project dependent- from 3.3V to 5V. In the case of Vcc being 3.3V (as is in my current case) then also the resistor values R2, R4, R6 and R10 need to be 0 Ohm.
For this specific mosfet level shifter it does not matter if the high level voltage is equal to the low level voltage since it is the difference between gate and source voltage that decides whether the mosfet switches on or not (the gate-source threshold voltage is 1.5V max per the datasheet).
Switching on however does not occur, even after:
1. changing the mosfet to another BSS138
2. changing R8 from 10k to 1k
3. verifying this board for manufacturing defects
4. reducing the SPI bus speed from default 8MHz to 1MHz
The circuit does work when I short drain and source. But obviously that defeats the purpose.
What should I be looking at to solve this case?
Board track lengths for this circuit are less than 1cm.
Vcc in my case is 3.3V so a level shifter is not technically required but the issue is that the high level MISO can change -project dependent- from 3.3V to 5V. In the case of Vcc being 3.3V (as is in my current case) then also the resistor values R2, R4, R6 and R10 need to be 0 Ohm.
For this specific mosfet level shifter it does not matter if the high level voltage is equal to the low level voltage since it is the difference between gate and source voltage that decides whether the mosfet switches on or not (the gate-source threshold voltage is 1.5V max per the datasheet).
Switching on however does not occur, even after:
1. changing the mosfet to another BSS138
2. changing R8 from 10k to 1k
3. verifying this board for manufacturing defects
4. reducing the SPI bus speed from default 8MHz to 1MHz
The circuit does work when I short drain and source. But obviously that defeats the purpose.
What should I be looking at to solve this case?
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) try checking the BSS138, the component tester will not only tell you it's basic specifications (and if it's working OK), but will tell you which pin is which. Not all devices have the same pinout, often different manufacturers use different pins - been there, been caught!.
