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Intel
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82845 MCH for SDR Datasheet 137
8 Testability
In the MCH, testability for Automated Test Equipment (ATE) board-level testing has been
implemented as an XOR chain. An XOR-tree is a chain of XOR gates, each with one input pin
connected to it (see Figure 10).
Figure 10. XOR Tree Chain
Input
XOR
Out
xor.vsd
Input Input Input Input
VCC1_8
The algorithm used for in-circuit test is as follows:
• Drive all input pins to an initial logic level 1. Observe the output corresponding to scan chain
being tested.
• Toggle pins one at a time (starting from the first pin in the chain and continuing to the last
pin) from its initial logic level to the opposite logic level. Observe the output changes with
each pin toggle.
8.1 XOR Test Mode Initialization
XOR test mode can be entered by pulling three shared pins (reset straps) low through the rising
transition of RSTIN#. The signals that need to be pulled are as follows:
• G_GNT# = 0 (Global strap enable)
• SBA1 = 0 (XOR strap)
• ST2 = 0 (PLL Bypass mode; it is recommended to enter PLL Bypass in XOR test mode)