U.S. stock futures slipped into negative territory Friday morning as an above-consensus 0.3% gain in the core consumer-price index was seen as giving the Federal Reserve more ammunition to raise interest rates in September. Mostly upbeat corporate earnings from John Deere ( DE ) and Hewlett Packard ( HPQ ) also contributed to the firmer tone before equities surrendered their minimal gains after the CPI data. The nominal index was only 0.1% higher in April, matching Wall Street expectations.