AN 100: In-System Programmability Guidelines

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

1.1.4. MultiVolt Devices and Power-Up Sequences

For the JTAG circuitry to operate correctly during in-system programming or boundary-scan testing, all devices in a JTAG chain must be in the same state. If you do not hold the JTAG pins in the test-logic-reset state, in-system programming errors can occur.

Therefore, in systems with multiple power supply voltages, you must hold the JTAG pins in the test-logic-reset state until all devices in the chain are fully powered up. This procedure is important because systems that have multiple power supplies cannot power all voltage levels simultaneously.

MAX devices have the MultiVolt™ feature and can use more than one power supply voltage:

  • MAX 10 devices—VCC/VCC_ONE provides power to core and periphery, VCCIO provides power to input pins and output buffer, and VCCA provides power to PLL regulator.
  • Other MAX devices—VCCINT and VCCIO for each I/O bank. VCCINT provides power to the JTAG circuitry; VCCIO provides power to input pins and output drivers for output pins, including TDO pin.

VCCINT Powered before VCCIO

If VCCINT is powered up before VCCIO, the JTAG circuitry is active but unable to drive out signals. Thus, any transition on the TCK pin can cause the state machine to transition to an unknown JTAG state. If TMS and TCK pins are connected to VCCIO and VCCIO is not powered up, the JTAG signals are left floating. These floating values can cause the device to transition to unintended JTAG states, leading to incorrect operation when VCCIO is finally powered up. Therefore, you must disable all JTAG signals.

VCCIO Powered before VCCINT

If VCCIO is powered up before VCCINT, the JTAG circuitry is not active but the TDO pin is tri-stated. Although the JTAG circuitry is not active and if the next device in the JTAG chain is powered up with the same trace as VCCIO, its JTAG circuitry must stay in the test-logic-reset state. Because all TMS and TCK signals are common, you must disable these signals for all devices in the chain. Therefore, you must disable the JTAG pins by pulling the TCK signal low and the TMS signal high. With the MAX 10 device hot-socketing feature, you no longer need to ensure a proper power up sequence for each device on the board.