Bus Protocol
E-4
E.2 Bus Protocol
The IEEE 1149.1 specification covers the requirements for the test access port
(TAP) bus slave devices and provides certain rules, summarized as follows:
-
The TMS and TDI inputs are sampled on the rising edge of the TCK signal
of the device.
-
The TDO output is clocked from the falling edge of the TCK signal of the
device.
When these devices are daisy-chained together, the TDO of one device has
approximately a half TCK cycle setup time before the next device’s TDI signal.
This timing scheme minimizes race conditions that would occur if both TDO
and TDI were timed from the same TCK edge. The penalty for this timing
scheme is a reduced TCK frequency.
The IEEE 1149.1 specification does not provide rules for bus master (emula-
tor) devices. Instead, it states that the device expects a bus master to provide
bus slave compatible timings. The XDS510 provides timings that meet the bus
slave rules.
