BYPASS

Putting a device into BYPASS mode effectively removes it from the JTAG chain. The device exhibits its normal (i.e. non-JTAG) behaviour, and places a 1-bit register in the boundary scan chain. This register is read only and always reads 0. Thus the functional core of the device is connected directly to its external pins, and the JTAG scan chain is entirely disconnected from both.

In BYPASS mode therefore,

• there is no JTAG control over the device output.
• it is not possible to read any information from the device.

It can be useful to place a device into BYPASS mode whenever the device is not the current focus of interest. This results in a shorter boundary scan chain, allowing more rapid updates of the status of other devices in the JTAG chain. However note that programmed devices, being free to operate normally, may boot up and begin to operate when in BYPASS mode.