MIPS cJTAG User Manual
MIPS® cJTAG Adapter User’s Manual
1 Introduction
MIPS provides an IEEE 1149.1-compatible JTAG debug and control port called EJTAG for its processor cores.
Recently,an updated IEEE standard, 1149.7, has been published. One of the enhancements is a reduction in the number of external signals required from four to two. For some chip designs, pin count is critical, and the ability to provide debugging capabilities with only two pins could be crucial.
MIPS provides a cJTAG Adapter IP block that converts a 2-pin 1149.7 (also known as cJTAG) to the 4-pin 1149.1
debug interface present on MIPS cores. The IP resides outside the core and is treated as a separate IP block from the
point of view of design verification and implementation.
MIPS debug probes will be enhanced to support both cJTAG and legacy EJTAG. EJTAG and cJTAG use the same
14-pin connector called out in the MIPS EJTAG Specification, but when connected to cJTAG, the TDI and TDO signals are not used.
The IEEE1149.7 Specification is complex and much more flexible than is needed in this application. The IP implemented here is a subset of 1149.7.
2 Overview
The cJTAG Adapter provides a probe interface consisting of two signals—TCKC and TMSC—and a device interface
consisting of four signals—SYS_TCK, SYS_TMS, SYS_TDI, and SYS_TDO. The TMSC signal is bidirectional, so
the Adapter separates the signal into three ports—TMSC_IN, TMSC_OUT, and TMSC_EN—and requires the system designer to provide the appropriate attachment to a bidirectional device pin. TCKC is sourced from the probe
(called DTS in the IEEE Specification). TMSC is bidirectional and carries control information to the Adapter and
data in both directions.
nTRST
TCKC
TMSC
nSP
Figure 1 cJTAG High Level Pins
nTRST
SYS_TCK
SYS_TMS
cJTAG Adapter
SYS_TDI
SYS_TDO
2 MIPS® cJTAG Adapter User’s Manual, Revision 01.00
3 Protocols
3.1 Online/Offline
The Adapter can be either online or offline. When the Adapter is offline, activity on the TCKC and TMSC signals
does not affect the 1149.1 port. When online, TCKC and TMSC indirectly drive the 1149.1 port to perform JTAG
scans. There is a protocol to switch between the online and offline states.
When the Adapter is reset, it is in the offline state. A reset can be performed using the optional nTRST signal or
through a sequential protocol on TCKC/TMSC. Switching online or offline and performing reset are accomplished
using an Escape sequence described in the IEEE 1149.7 Specification. While TCKC is held high, TMSC is toggled a
certain number of times. The Adapter keeps a count of the number of edges observedon TMSC and executes the corresponding command at the next TCKC rising edge.
TCKC
TMSC
Figure 2 cJTAG Online
As described in the IEEE Specification, there may be a TMSC edge coincident with the last rising edge of TCKC
before the Escape sequence begins, and that edge may or may not be detected by the Escape logic, depending on signal skew. A single pulse that would normally be two edges could therefore be counted as three edges. Escape detection logic takes this possibility into account. The Adapter interprets TMSC edges in an Escape as described in the
following subsection.
3.2 Online Activation Code
Following the Online Escape sequence, theprobe transmits an Online Activation Code (OAC), ExtensionCode (EC),
and Check Packet (CP), for a total of 12 TCKC pulses. The Adapterobserves the control data in these codes and activates only if the requested protocol variations are supported by the Adapter. In this implementation, only one form of
activation code is supported; any other sequence of control bits will return the Adapter to the offline state.
Referring to the IEEE Standard, the OAC required is 1100, transmitted LSB first, which connotes TAP.7 star-2 scan
topology. The EC must be 1000, indicating the short form and use of the Run-Test/Idle TAP state when switching
online or offline. The CP is 0000. At the rising edge of TCKC in the last bit of the CP, the Adapter is activated. From
that point forward, activity on TCKC/TMSC is interpreted as Oscan1 format, described in the IEEE Standard, until
the Adapter is reset or otherwise taken offline.
MIPS® cJTAG Adapter User’s Manual, Revision 01.00 3
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