3.1. Mictor 38-bit Debug and Trace Connector

Mictor-38 connector as defined by MIPI Alliance has all signals from MIPI20 connector and adds up to 16 bits of parallel trace and defines more trigger pins. Mictor-38 connector is also designed for high-speed trace (it is rated for 400MHz double edge captures).

Mictor-38 connector provides also an option to have different reference voltages for debug and trace.

Table 2. Mictor-38 Connector Layout
Signal Ref Voltage Odd Pin# Even Pin# Ref Voltage Signal

NC

1

2

NC

NC

3

4

NC

GND

5

6

Trace

TRC_CLK

TRIGIN

Debug

7

8

Debug

TRIGOUT

nRESET

Debug

9

10

Trace

EXTTRIG

TDO

Debug

11

12

Trace

VREF_TRACE

GND

13

14

Debug

VREF_DEBUG

TCK / TCKC

Debug

15

16

Trace

TRC_DATA[7]

TMS / TMSC

Debug

17

18

Trace

TRC_DATA[6]

TDI

Debug

19

20

Trace

TRC_DATA[5]

nTRST

Debug

21

22

Trace

TRC_DATA[4]

TRC_DATA[15]

Trace

23

24

Trace

TRC_DATA[3]

TRC_DATA[14]

Trace

25

26

Trace

TRC_DATA[2]

TRC_DATA[13]

Trace

27

28

Trace

TRC_DATA[1]

TRC_DATA[12]

Trace

29

30

Trace

Logic '0' (GND)

TRC_DATA[11]

Trace

31

32

Trace

Logic '0' (GND)

TRC_DATA[10]

Trace

33

34

Trace

Logic '1'

TRC_DATA[9]

Trace

35

36

Trace

EXT / TRC_CTL

TRC_DATA[8]

Trace

37

38

Trace

TRC_DATA[0]
Above table is using names compatible with MIPI specification (however MIPI specification shows rows of pins starting from 38 down to 1).

3.1.1. Explanation for additional pins (comparing to MIPI20)

All debug signals share alternate functions as defined for the MIPI20 connector.

Table 2. Micror-38 additional pins (comparing to MIPI20 defined above)
Pin# Pin Name Explanation (comparing to MIPI20)

7

TRIGIN

Same as MIPI20 #18 alternative pin function but not shared with trace.

8

TRIGOUT

Same as MIPI20 #20 alternative pin function but not shared with trace.

10

EXTTRIG

External trace trigger from target (some trace probes may use it).

21

nTRST

Same as MIPI20 #16 alternative pin function but not shared with trace.

36

EXT / TRC_CTL

Not applicable (should be 0). May be also used to denote valid/idle state, but it may not be supported by all trace probes.

3.1.2. Dual voltage (different for debug and different for trace) configurations

Sometimes (due to speed reasons) it may be beneficial to drive SoC trace pins with different (usually lower) voltage then the debug signals. Such a configuration may be supported using a single Mictor connector or two connectors (Mictor for trace only and MIPI for debug only). Be aware that two different voltages may not be supported by simpler trace probes.

Single voltage - single Mictor (Recommended)

  • Mictor #12: VREF_TRACE=VREF_DEBUG (Required)

  • Mictor #14: VREF_DEBUG (Recommended, see NOTE *1 below) or NC

Single voltage - trace via Mictor, debug via extra JTAG connector (NOT Recommended)

  • Mictor #12: VREF_TRACE=VREF_DEBUG (Required)

  • Mictor #14: NC (Recommended, see NOTE #1 below) or VREF_DEBUG

  • Mictor JTAG pins: Connected or NC (Recommended, see NOTE #2 below)

  • JTAG connector VTREF (#1): VREF_DEBUG (Required)

  • JTAG connector JTAG pins: Connected (Required)

Dual voltage - single Mictor (NOT Recommended)

  • Mictor #12: VREF_TRACE (Required)

  • Mictor #14: VREF_DEBUG via jumper on PCB (Required, see NOTE #3 below)

Dual voltage - trace via Mictor, debug via extra connector (Recommended)

  • Mictor #12: VREF_TRACE (Required)

  • Mictor #14: NC (Required, see NOTE #3 below)

  • Mictor JTAG pins: NC (Required, see NOTE #4 below)

  • JTAG connector VTREF (#1): VREF_DEBUG (Required)

  • JTAG connector JTAG pins: Connected (Required)

#1 Jumper (on PCB) between Mictor pin#14 and VREF_DEBUG rail on PCB can be used to select NC or VREF_DEBUG. Some trace probes (such as TRACE32 from Lauterbach) require VTREF_DEBUG to be present on pin #14.
#2 If JTAG pins are NC, JTAG quality/speed may be better as there will be no stubs introduced by extra routing on PCB.
#3 Jumper provides extra safety in case a trace probe/adapter which does not support dual voltage is used. Before fitting this jumper, make sure the probe/adapter you are using is NOT shorting Mictor pin#12/#14 internally. If this is the case, two voltage rails may be shorted and the target may be permanently damaged. Some trace probes (such as TRACE32 from Lauterbach) require VTREF_DEBUG to be present on pin #14.
#4 All JTAG pins should be NC for a reason mentioned in NOTE 2. But mainly to make sure that there will be only a single voltage present on this connector.

EXTRA NOTES (related to debug and trace voltages)

  1. Lower voltage allows faster trace, but it is then more critical to have correct PCB design.

  2. Allowed reference voltage ranges (for JTAG and trace) are different for different probes.

  3. Lower voltage used for trace may be a good choice with FPGA-based development boards.

    • Trace pins may be available on an FPGA bank, which is setup for lower IO voltage.

  4. When high-speed trace is important Mictor-38 should be the only debug and trace connector on the PCB.

    • In case two connectors are used, trace signals should have routing priority.

    • Many probe vendors provide adapters from Mictor to standard JTAG-only connectors, so non-trace probes can be used with target/PCB with Mictor-only connector.

  5. Not all trace probes which support the Mictor-38 connector are capable of handling dual voltage tracing.

    • At the moment of this writing at least I-jet-Trace-A/R/M (by IAR Systems) and Trace32 (by Lauterbach) probes support such a mode (in both single Mictor and two Mictor + JTAG connectors).

  6. It is not recommended to add buffers on PCB to adjust JTAG (usually higher) voltage to trace voltage.

    • It not only affects signal quality but also introduces extra delays, which may create problems for simple probes.

    • It is very hard to properly handle fast switching of bidirectional signals, so cJTAG and SWD debug protocols may never reliably work.

    • It makes PCB more complicated without good reason.

3.1.3. Explanation for Mictor-38 pins #30/32/34/36

It may be hard to understand why TRC_DATA[0] is not together with other TRC_DATA[1..15] signals and why pins #30/32/34 have specific fixed values (Logic '0' or Logic '1').

This is caused by the desire to provide compatibility with initial versions of Arm trace. These older versions used these 4 pins to denote idle state. Modern trace probes ignore these signals, but just in case they do not, it is better and safer to provide logic level as above. As TRC_CTL is not used, it should be tied to 0 on PCB but may be optionally used as an extra external trace trigger (from target to probe).