Motorola Command One User Manual download pdf (Page 13)

TCK Drive and Timing Considerations

Motorola Functional Description 2-3

2.2 Multiple Target Connections

Multiple target devices may be connected in series, allowing a single command converter,

JTAG/OnCE connector to control multiple devices, as in Figure 2-3. Data flows from the

JTAG host, into each JTAG implementation through TDI, out through TDO and into TDI

in the next chip, eventually returning to the JTAG host.

Figure 2-3. Multiple JTAG Target Connections (Serial Method)

2.3 TCK Drive and Timing Considerations

The signals from the command converter are TDO, TCK and TMS, and TRST

. The TCK

signal requires fast rise and fall times dictated by the TCK pin timing specification, and

consequently attention must be given to the drive capability of the circuits driving this

signal.

When driving the TCK and TMS circuits with a large number of target devices, the user

must pay attention to the rise and fall times of TCK and TMS. Excessive capacitance may

cause communication problems when driving a single circuit that connects multiple TCK

or multiple TMS input pins. Excessive capacitance can also cause communication

problems with a single circuit connecting multiple TMS input pins.

Acceptable transition times may be achieved for TCK and TMS by driving no more than

four JTAG inputs from each buffered output. This may be achieved with two

configurations.

Figure 2-3 shows one method. Here (in effect) one signal connects each of the TCK

inputs, and one signal connects each of the TMS inputs. A buffer is placed in the circuit

after each fourth input at most, to restore the signal quality for subsequent inputs. The

propagation delay of the buffer is not significant.

TDI TDO

TCK TMS

TDI TDO

TCK TMS

TDI TDO

TCK TMS

TDI TDO

TCK TMS

TDI TDO

TCK TMS

TDI TDO

TCK TMS

TCK

TMS

TDO

TDI

Maximum of 4 Loads on

TCK Circuits

Buffer—74HCT244

or Similar

RESET

TRST

TRST RESET

AA2039

eescale S

emiconduct

, I

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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Motorola Command One User Manual Page 13

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