Microchip Technology MEC172 Series Layout Manual page 24

AN3759
3.0 JTAG DESIGN AND LAYOUT GUIDE
This section provides general hardware information for using the MEC1721/MEC1723 JTAG interface and working with
JTAG master and slaves.
This document includes the following topics:
• Section 3.1, "MEC1721/MEC1723 JTAG Capabilities," on page 24
• Section 3.2, "General PCB Layout Considerations for JTAG," on page 24
• Section 3.3, "Pin Connections," on page 24
• Section 3.4, "JTAG Internal Pull-Up," on page 27
• Section 3.5, "JTAG Reset," on page 27
3.1 MEC1721/MEC1723 JTAG Capabilities
MEC1721/MEC1723 devices have the following debug capabilities:
• JTAG-Based DAP Port, Comprised of SWJ-DP and AHB-AP Debugger Access Functions
• Full DWT Hardware Functionality: 4 Data Watchpoints and Execution Monitoring
• Full FPB Hardware Breakpoint Functionality: 6 Execution Breakpoints and 2 Literal (Data) Breakpoints
• Accessed via 4-wire JTAG or 2-wire ARM SWD
• Comprehensive ARM-Standard Trace Support: Full DWT, ITM, ETM, TPIU functionalities
3.2 General PCB Layout Considerations for JTAG
Please follow the PCI Specification's Routing and Layout Guidelines for the JTAG interface signals to support the JTAG
interface speed up to 33MHz.
• In order to improve the clock transmission line's signal integrity, the following is recommended:
- Keep the clock traces as straight as possible.
- Use arc-shaped traces instead of right-angle bends.
- Do not use multiple signal layers.
- Do not use vias to reduce impedance change and reflection.
- Place a ground plane next to the outer layer to minimize noise effect.
- Terminate clock signals to minimize reflection.
• The JTAG cable that attaches to the MEC1721/MEC1723 motherboard has a standard 20-pin .1" spacing female
connector on it. Normally, the MEC1721/MEC1723 motherboard just has a 20-pin 0.1" spacing pin strip on the
board to mate with it.
• If the MEC1721/MEC1723 motherboard design does not have the space for a 20-pin male pin strip, then the board
designer can place a 6 pin header on the motherboard and build a 6-pin to 20-pin adapter cable to attach to the
20-pin female connector on the JTAG cable. This is shown in Figure 3-1, "6-Pin to 20-Pin Adapter Board (w/
BOM)".
3.3 Pin Connections
3.3.1 4-WIRE JTAG CONNECTION
Six signals are the minimum number required on the motherboard side; these are described in
in FIGURE 3-1: 6-Pin to 20-Pin Adapter Board (w/ BOM) on page
TABLE 3-1: MEC1721/MEC1723 4-WIRE JTAG PINS
Name
VTR
TDI
TMS
CLK
DS00003759B-page 24
The motherboard VTR is always 3.3V. It is recommended to add a 49-ohm series resistor
for motherboard protection. The JTAG cable senses the voltage level on this line and
drives the JTAG logic levels to the same voltage level from the target system.
Test Data In
Test Mode Select
Test Clock
25.
JTAG Cable Connection
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Table 3-1 and illustrated