nanoFIP JTAG feature
JC_TCK
In agreement with the users, the JC_TCK has a predefined frequency of 5 MHz*.
_The code has been written in a generic way that with a modification of
a constant we could change to 2.5 MHz or 1.25 MHz and tests on real
hardware have been performed for these speeds;
however, keep in mind that just changing clock frequency would
immediately invalidate all radiation testing done so far!_
During a reset, the JC_TCK stays in the inactive state.
TMS-TDI bits
nanoFIP checks if the concatenation of bytes 2 & 3 of the consumed var4
RP_DAT frame gives a number higher than 976; if so the complete
variable is ignored.
Note also that if the CRC, PDU_TYPE or LENGTH bytes of the frame are
wrong, the variable is also ignored.
TRST
Note that the TAP TRST signal of the target FPGA is not driven by
nanoFIP.
A
report
by NASA suggests that this input should always be grounded under
radiation.
For the reconfiguration of the FPGA, which should always take place off
beam, the TRST grounding should be released and then nanoFIP should
start receiving var4 frames driving the TAP state machine.
This additional logic before the start-up of the reprogramming adds
reliability in such a critical application.
Note: For the implementation of this TRST grounding-release logic, it may be useful to note that through Actel Designer you could select a guaranteed stable state of the FPGA outputs while reprogramming.
_Back to the Hints n' Tips Guide_
E.Gousiou, March 2012