ADS7128 Open-Drain GPIO Test
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CH0=push-pull HIGH, CH7=push-pull LOW

Test 1: Open-drain LOW sinks
  CH1=OD LOW, CH2 reads: 0.00 V
  PASS: CH2 < 2.0V

Test 2: Open-drain HIGH floats
  CH1=OD HIGH (float), CH2 reads: 3.56 V
  PASS: CH2 > 2.5V

Test 3: Push-pull vs open-drain
  CH1=PP LOW,  CH2: 0.00 V
  CH1=PP HIGH, CH2: 4.16 V
  CH1=OD HIGH, CH2: 3.56 V (from test 2)
  PP HIGH > OD HIGH: PASS

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RESULTS: 3/3 tests passed
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