AFE1_AFE2-1:1
I-jet
15
In a JTAG device that fully complies to IEEE1149.1 standard, the TDI/TMS signals
should be sampled on the rising edge of TCK, and TDO should be sent on the falling
edge of TCK. I-jet takes advantage of these requirements and changes its TDI and TMS
signals on the falling edge of TCK and samples the TDO on the rising edge of TCK.
However, to accommodate target boards with long JTAG chains and fast JTAG clocks,
I-jet allows TDO to be as late as 50 ns after the rising edge of TCK.
Note: In the adaptive mode of operation, I-jet samples the TDO on the rising edge of
RTCK rather than TCK.
This table shows the timing specifications of the JTAG port measured at the end of its
MIPI-20 cable without connection to target (VTref set to 3.3 V). The only load on the
measured signals is the oscilloscope 3.9 pF probe.
1 T
sod
is the maximum delay from the falling edge of TCK and a valid level on the I-jet
output signals, TDI and TMS. The target MCU will sample these signals on the
following rising edge of TCK and so the minimum setup time for the target, relative to
the rising edge of TCK, is T
bscl
-T
bsod
.
2 T
sid
is the minimum setup time for the TDO input signal, relative to the rising edge of
TCK when I-jet samples this signal. Because the target MCU changes its TDO value on
the previous falling edge of TCK, there might not be enough time at very-high JTAG
speeds for the TDO to arrive before the positive edge of TCK. To compensate for any
TDO delays, I-jet configures itself automatically to delays introduced to the TDO by the
target board and will tolerate TDO delays of up to 50 ns after the positive edge of the
TCK.
HARDWARE REVISION HISTORY
These are the versions of I-jet:
Parameter Min Max Description
T
tckl
15.6 ns 250 us TCK LOW period
T
tckh
15.6 ns 250 us TCK HIGH period
T
sod
1
-- 2.0 ns TDI and TMS outputs valid from TCK
falling
T
sid
2
3 ns before TCK to 50 ns
after TCK
-- TDO setup to TCK rising
T
tdo
T
tckl
-- TDO valid length
Table 1: I-jet JTAG port timing specifications
Version Change specification Date
Version A The first version April 2012
Table 2: I-jet versions
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