J-Link OB SAM3U NordicSemi

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VCOM functionality

This J-Link model supports VCOM.

Supported baudrates

This J-Link OB supports max. 1 MBaud. The internal divider is calculated as follows:

 Baudrate = 96000000 / Div * 16;
 Div = 96000000 / (Baudrate * 16);

Note: The real baudrate and the selected one should not deviate more than 3% to guarantee proper communication on VCOM.

Pin driving policy

The following pins are available on this OB:

  • OB_Tx (driven by J-Link OB)
  • OB_Rx (driven by target)
  • CTS (driven by target. LOW == Target ready to receive data)
  • RTS (driven by J-Link OB. LOW == J-Link OB ready to receive data)

J-Link OB starts driving its VCOM output pins as soon as it receives a "set baudrate" command. This command is send by the PC as soon as a COM port is opened. Unfortunately, there is no specific command / packet etc. send by the PC in case a COM port is opened. Therefore the "set baudrate" approach is the only one to detect COM port activity.

J-Link OB stops driving the RTS pin if J-Link OB detects that hardware flow control is not used on the target side.

Hardware flow control support

Hardware flow control is supported by this J-Link OB. By default, J-Link detects at runtime if the target uses flow control or not. This is done as follows:

  • If CTS is detected as HIGH (Target not ready) before the first character is sent by J-Link OB, the pin is assumed to be not used by the target and flow control on the J-Link side gets disabled (CTS is ignored from now on, J-Link stops driving RTS)

Force or disable hardware flow control

It is possible to configure J-Link OB to bypass the hardware flow control auto-detection in the firmware and assume that hardware flow control is always or never used. This can be done via J-Link Commander and the following command:

 SetHWFC Enable/Disable/Force

Note: This setting gets active immediately and survives a power-cycle of J-Link.

  • Enable: Default. Enables hardware flow control and runtime auto-detection
  • Disable: Disables hardware flow control (never used)
  • Force: Enables hardware flow control and bypasses runtime auto-detection (RTS is kept driven and CTS is kept sampling)
Force hardware flow control

Minimum requirements