AN 100: In-System Programmability Guidelines

ID 683546
Date 9/22/2014
Document Table of Contents

1.1.6. Pull-Up and Pull-Down of JTAG Pins During In-System Programming

An Altera device operating in in-system programming mode require four pins: TDI, TDO, TMS, and TCK. Three of the four JTAG pins have internal weak pull-up or pull-down resistors.

The TDI and TMS pins have internal weak pull-up resistors, while the TCK pin has an internal weak pull-down resistor. However, for device programming in a JTAG chain, there might be devices that do not have internal pull-up or pull-down resistors. Altera recommends to pull the TMS signal high externally through 10-kΩ and the TCK signal low through 1-kΩ resistors. Pulling up the TDI signal externally for the device is optional.

Figure 1. External Pull-Up and Pull-Down Resistors for TMS and TCK of a JTAG Chain in Altera DevicesFigure shows the external pull-up and pull-down for the TMS and TCK pins of the JTAG chain in Altera devices. The TDO pin does not have internal pull-up or pull-down resistors, and does not require external pull-up or pull-down resistors.

The TMS signal is pulled high so that the test access port (TAP) controller remains in the TEST_LOGIC or RESET state even if there is input from TCK signal. During power up, you must pull the TCK signal low to prevent the TCK signal from pulsing high. Pulling the TCK signal high is not recommended because the increase in power supply to the pull-up resistor causes the TCK signal to pulse high; therefore, it is possible for the TAP controller to reach an unintended state.