Revisions Affected

0, A

Details

The combinations of use cases for a specific analog input pin that need special considerations are shown in Table 3-2. As shown in this table, special considerations or workarounds need to be used for the combination of CMPSS Input, ADC Sampling, and AGPIO.

The AGPIO analog pin path contains an extra series switch of 53Ω. This creates a low-capacitance isolated node shared by the ADC and CMPSS comparator, as shown in Figure 3-1. This node can be disturbed when the ADC samples the channel (depending on the prior voltage stored on the ADC sample-and-hold capacitor), and this disturbance can cause a false CMPSS event of up to 50ns. To accommodate this potential disturbance, the workarounds below can be implemented.

Figure 3-1 Analog Subsystem Diagram with AGPIO and AIO Analog Pin Types

Workarounds

1. Use a different pin (that is AIO pin type) for analog channels that need both ADC and CMPSS together.
2. Use the CMPSS Digital Filter with a setting of 50ns or greater, which will filter the temporary disturbance.
3. Pre-condition the sample-and-hold capacitor of the ADC so that the disturbance will not cause a false trip. For example, perform a dummy read of a 3.3V connection from a different channel on the ADC immediately before the impacted channel is read, so the disturbance is in the positive direction, away from the false trip. The opposite dummy read of a 0V signal would be used if the false trip is inverted in polarity.