The Four-Wire interface uses standard JTAG (IEEE® 1149.1-2001)
interface signals.
Test Clock (TCK) – Drives data
in/out
Test Mode Select (TMS) – Selects
operational mode
Test Data Input (TDI) – Data into
the device
Test Data Output (TDO) – Data out
of the device
Since only one data line is available, the protocol is necessarily serial, similar to
Serial Peripheral Interface (SPI). The clock input occurs at the TCK pin. Configuration
is performed by manipulating a state machine bit by bit through the TMS pin. One bit of
data is transferred in and out per TCK clock pulse at the TDI and TDO pins. Different
instruction modes can be loaded to read the chip ID or manipulate chip functions.
Data presented to TDI must be valid for a chip-specific setup time before, and hold time,
after the rising edge of the TCK. TDO data is valid for a chip-specific time after the
falling edge of TCK, see Figure 5-3.Figure 5-3. Four-Wire JTAG
Interface
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