Impedance matching between device pins and transmission lines on the board is conventionally done using separate resistors in each trace. The board space required, would then, be unwarranted. The VIM is a method by which we match the impedance of a group of device pins (I/O drivers) with their corresponding signal trace using a single resistor. VIM eliminates the transmission line effects improving signal integrity, I/O bandwidth and system reliability without excessive board space.
How VIM works.
The calibrating resistor is connected between pins ZQ<1:0> (ZQ in case of 18M) on the corresponding port and ground. The resistor must be 5 times larger than the intended line impedance driven by the dual port. The VIM or VIS circuit can set the impedance of both or either ports using the corresponding ZQ<1:0> pins. The two pins must be tied to the same setting.
The configured ports' O/P impedance is corrected for drifts in V and T every 1024 clock cycles and in case of significant changes in V and T the recalibration period is multiples of 1024. The VIM/VIS circuit gets reset when the master reset is asserted and takes 1024 cycles to update after a reset operation. To disable VIM, pin ZQ is connected to Vdd and with this all the output drivers are turned on and the output is configured for minimum impedance. Using VIM the impedance of all the output pins can be configured except the JTAG.
For accurate and reliable operation the resistor values and tolerances should be as given:
Resistor values: 100-350 W, tolerance ?2%, temperature co-efficient: 200pmm/?C
The impedance at the ports should be as given:
O/P impedance: 20-75 W with a ?15% tolerance.
The Application Note for Variable Impedance mathcing with Fullflex Dual Ports can be accessed here: Using Variable Impedance Matching with FullFlex Dual-Ports