idontwantausername
Member
Something strange is happening and I'm hoping you can help me.
There are two boards -- one with an MCP2551 and other with a SN65 and Teensy 3.2, they are connected by a 4 wire cable.
Normally Teensy 3.2 and SN65 have a serial between them, but for this example it is disconnected. They only connections between the Teensy and the outside world are: Vin (5V), GND, 3V to SN65.
When power is connected to the 48V connector, Teensy 3.2 fails to boot up... Unless any of the following are true:
1. MCP 2551 is off when Teensy 3.2 is just booting up
2. 3.3V pin of the Teensy is disconnected from SN65
3. Both CAN_H and CAN_L are disconnected between MCP2551 and SN65
4. While 48V is connected, GND between the two boards is disconnected! CAN_H/L is somehow acting as ground?
5. Using Teensy LC instead of 3.2 (I know LC doesn't have a CAN module)
Other facts:
- 48V, 5V and 3.3V rails come on smoothly without any spikes, the rise time is around 4ms
- SN65 is drawing 8-14mA from the Teensy
- There are current spikes on the 48V line during startup (because of input capacitance on the second PCB)
To summarize, if MCP 2551 and SN65 are booting up at the same time, Teensy doesn't boot up. Even though there is nothing suspicious on either 5V or 3.3V buses.
There are two boards -- one with an MCP2551 and other with a SN65 and Teensy 3.2, they are connected by a 4 wire cable.
Normally Teensy 3.2 and SN65 have a serial between them, but for this example it is disconnected. They only connections between the Teensy and the outside world are: Vin (5V), GND, 3V to SN65.
When power is connected to the 48V connector, Teensy 3.2 fails to boot up... Unless any of the following are true:
1. MCP 2551 is off when Teensy 3.2 is just booting up
2. 3.3V pin of the Teensy is disconnected from SN65
3. Both CAN_H and CAN_L are disconnected between MCP2551 and SN65
4. While 48V is connected, GND between the two boards is disconnected! CAN_H/L is somehow acting as ground?
5. Using Teensy LC instead of 3.2 (I know LC doesn't have a CAN module)
Other facts:
- 48V, 5V and 3.3V rails come on smoothly without any spikes, the rise time is around 4ms
- SN65 is drawing 8-14mA from the Teensy
- There are current spikes on the 48V line during startup (because of input capacitance on the second PCB)
To summarize, if MCP 2551 and SN65 are booting up at the same time, Teensy doesn't boot up. Even though there is nothing suspicious on either 5V or 3.3V buses.