Instead of waiting for a signal to propagate through the signal processing blocks, the threshold feature provides an early indication of the incoming signal level. This early indication of the signal level can be used by the automatic gain control (AGC) implemented in the PL. Threshold levels used to indicate the input signal level are set using the RFdc driver API.
Threshold monitoring occurs when the RF-ADC sampled data enters the datapath. This data is compared to a user-defined threshold. A threshold status signal is sent to the outputs on the IP core, to indicate that a user-defined threshold has been exceeded. There are two real-time over threshold output signals for each RF-ADC. The modes of the threshold monitoring circuit are listed in the following table.
| Mode | Description |
|---|---|
| Off | The threshold circuit is disabled and the status outputs are Low. |
| Sticky over | The threshold status signal is High when the data from the RF-ADC exceeds the programmed upper threshold value. The status is kept until a clear action is sent. 1 |
| Sticky under | The threshold status signal is High when the data from the RF-ADC remains below the programmed lower threshold value for the duration of a user-specified time or delay. The status is kept until a clear action is sent 1 . This delay value for the lower threshold is defined by a 32-bit counter. The counter is set using the RFdc driver API. Using this mechanism prevents short duration excursions triggering a threshold event. |
| Hysteresis | The status output is set when the programmed upper threshold value is exceeded, and is cleared when the signal remains below the lower threshold value for the duration of a user-specified delay value. This delay value for the lower threshold is defined by a 32-bit counter. The counter is set using the RFdc driver API. The delay adds hysteresis to the threshold detection to prevent short duration excursions triggering a threshold event. |
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Threshold levels are set as 14-bit unsigned values, with any value from 0 to 16383 allowed. The maximum value, 16383 represents the absolute value of the full-scale input of the RF-ADC. The 32-bit programmable delay counts RF-ADC samples. To relate this count to a specific time, the following equation can be used:
Counter Value = (delay(ms) x ADCSampleRate (GSPS) x 106) / Interleaving Factor
where the Interleaving Factor is 4 for the Quad RF-ADC tile and is 8 for the Dual RF-ADC tile.
The delay can also be expressed in samples instead of ms as below:
Counter Value = delay (RF-ADC samples) / Interleaving Factor = delay (sub-ADC samples)