Occasional ContributorCyclone IV JTAG pins connected to VCCA (2.5V) with IO bank of 3.3V
Hello
I am using Cyclone IV GX (EP4CGX150CF23C8N).
JTAG and configuration device (AS - EPCQ128ASI16N) connection done as described on figure 8-29 (page 220 of Cyclone IV device handbook Volume 1).
All VCCIO in my project are 3.3V.
I am wondering is it correct to connect TMS, TDI (with pull-up resistors) and JTAG connector pin 4 (for USB/BYTE blaster) to VCCA (2.5V) while VCCIO = 3.3V?
In page 210 of handbook it is also mentioned:
"All the JTAG input pins are powered by the VCCIO pin and support the LVTTL I/O
standard only."
Thank you
For 3.3V LVTTL, the spec for VIH(min) is 1.7V and VIH(max) is 3.6V. So a pullup to a voltage between these limits (2.5V) is OK.
Normally tho one would connect the pullup to the same voltage source as drives the VCCIO for those I/O pins.
The only concern would be what are the powerup characteristics of the 2.5V and 3.3V rails in your system?
Do they powerup simultaneously, or is there a significant time differential in their ramp rates?
For example, if the 2.5V rail turns on significantly later, you could get into the situation where the pullup to 2.5V is actually a pulldown.
Personally I would connect the pullups to the same voltage source that drives the VCCIO pin for those I/Os.
