8.7.9. Scan chain 3

This scan chain allows ARM7DMT to control an external boundary scan chain. Scan chain 3 is provided so that an optional external boundary scan chain may be controlled via ARM7DMT. Typically, this would be used for a scan chain around the pad ring of a packaged device. Its length is user‑defined.

The following control signals are provided. These are generated only when scan chain 3 has been selected. These outputs are inactive at all other times.

DRIVEBS

This would be used to switch the scan cells from system mode to test mode. This signal is asserted whenever either the INTEST, EXTEST, CLAMP or CLAMPZ instruction is selected.

PCLKBS

This is an update clock, generated in the UPDATE-DR state. Typically the value scanned into a chain would be transferred to the cell output on the rising edge of this signal.

ICAPCLKBS ECAPCLKBS

These are capture clocks used to sample data into the scan cells during INTEST and EXTEST respectively. These clocks are generated in the CAPTURE-DR state.

SHCLKBS SHCLK2BS

These are non-overlapping clocks generated in the SHIFT-DR state used to clock the master and slave element of the scan cells respectively. When the state machine is not in the SHIFT-DR state, both these clocks are LOW.

nHIGHZ

This signal may be used to drive the outputs of the scan cells to the high impedance state. This signal is driven LOW when the HIGHZ instruction is loaded into the instruction register, and HIGH at all other times.

External scan chains

In addition to the above control outputs, the following are provided for use when an external scan chain is in use:

SDINBS output

should be connected to the serial data input.

SDOUTBS input

should be connected to the serial data output.

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