JTAG接口PCB设计要点.docVIP

JTAG接口PCB设计要点 1、 Keep JTAG traces as short as possible. 2、 Either galvanically isolate JTAG ground from offboard equipment or make sure that the PCB and the off-board equipment share a common ground. 3、 Add a 3–5 kΩ pull-up resistor to the JTAG interface’s TCK pin to reduce susceptibility to EMI. 4、 In noisy systems, add pull-down or pull-up resistors to every JTAG signal. 5、 Place small series resistance on JTAG traces that are routed to external connectors. 4.3.2. The JTAG Interface Because JTAG signals have only a weak on-chip pull-up resistance and are not deglitched, signal traces are particularly susceptible to noise coupling and EMI. To reduce EMI sensitivity, JTAG traces should be kept as short as possible. If the PCB is not galvanically isolated from the off-board equipment (see 5. Isolation And Protection on page 11), the PCB and equipment must share a common ground, which can be established by connecting a pin of the JTAG header to the PCB ground plane. The JTAG signals can be made more immune to noise by adding some passive circuitry. External pull-up or pull-down resistors can be added to aid the relatively weak on-chip pull-ups. Most applications will be sufficiently protected by adding a 3–5 kΩ pull-up resistor to the TCK signal. If the device is to be used in a particularly noisy environment, all JTAG signals should have strong external pull-ups or pull-down circuits to digital ground. Please note that placing a pull-down resistor on TCK will make your hardware incompatible with the USB Debug adapter. Capacitive ringing across long JTAG cables can cause communication difficulties. Placing a small series resistance on JTAG signals dampens this ringing and improve performance. Silicon Laboratories MCU target boards use a 5x2 header. Figure 11 shows a circuit diagram for the header, along with connections for a JTAG device.