Hardware Description

 

 

Table 3-4 JTAG signal description (continued)

 

 

 

Name

Description

Function

 

 

 

RTCK

Return TCK

Some devices sample TCK (for example a synthesizable core

 

(to JTAG equipment)

with only one clock), and this has the effect of delaying the time

 

 

at which a component actually captures data. RTCK is a

 

 

mechanism for returning the sampled clock to the JTAG

 

 

equipment, so that the clock is not advanced until the

 

 

synchronizing device captured the data. In adaptive clocking

 

 

mode, Multi-ICE is required to detect an edge on RTCK before

 

 

changing TCK. In a multiple device JTAG chain, the RTCK

 

 

output from a component connects to the TCK input of the

 

 

down-stream device. The RTCK signal on the module

 

 

connectors HDRB returns TCK to the JTAG equipment. If there

 

 

are no synchronizing components in the scan chain then it is

 

 

unnecessary to use the RTCK signal and it is connected to

 

 

ground on the motherboard.

 

 

 

TCK

Test clock

TCK synchronizes all JTAG transactions. TCK connects to all

 

(from JTAG equipment)

JTAG components in the scan chain. Series termination resistors

 

 

are used to reduce reflections and maintain good signal integrity.

 

 

TCK flows down the stack of modules and connects to each

 

 

JTAG component. However, if there is a device in the scan chain

 

 

that synchronizes TCK to some other clock, then all down-stream

 

 

devices are connected to the RTCK signal on that component

 

 

(see RTCK).

 

 

 

TDI

Test data in

TDI goes down the stack of modules to the motherboard and then

 

(from JTAG equipment)

back up the stack, labelled TDO, connecting to each component

 

 

in the scan chain.

 

 

 

TDO

Test data out

TDO is the return path of the data input signal TDI. The module

 

(to JTAG equipment)

connectors HDRB have two pins labelled TDI and TDO. TDI

 

 

refers to data flowing down the stack and TDO to data flowing up

 

 

the stack. The JTAG components are connected in the return path

 

 

so that the length of track driven by the last component in the

 

 

chain is kept as short as possible.

 

 

 

TMS

Test mode select

TMS controls transitions in the tap controller state machine.

 

(from JTAG equipment)

TMS connects to all JTAG components in the scan chain as the

 

 

signal flows down the module stack.

 

 

 

3-26

© Copyright ARM Limited 1999. All rights reserved.

ARM DUI 0125A