User Manual

Apollo3 Blue Datasheet

DS-A3-0p9p1 Page 731 of 909 2019 Ambiq Micro, Inc.

18.2.9 Window Comparator

A window comparator is provided which can generate an interrupt whenever a sample is determined to be

inside the window limits or outside the window limits. These are two separate interrupts with separate

interrupt enables. Thus one can request an interrupt any time a specified slot makes an excursion outside

the window comparator limits.

The window comparison function has an option for comparing the contents of the limits registers directly

with the FIFO data (default) or for scaling the limits register depending on the precision mode selected for

the slots.

Firmware has to participate in the determination of whether an actual excursion occurred. The window

comparator interrupts set their corresponding interrupt status bits continuously whenever the inside or

outside condition is true. Thus if one enables and receives an “excursion” interrupt then the status bit can’t

be usefully cleared while the ADC slot is sampling values outside the limits. That is, if one receives an

excursion interrupt and clears the status bit, it will immediately set again if the next ADC sample is still

outside the limits. Thus firmware should reconfigure the interrupt enables upon receiving an excursion

interrupt so that the next interrupt will occur when an ADC sample ultimately goes back inside the window

limits. Firmware may also want to change the windows comparator limit at that time to utilize a little

hysteresis in these window comparator decisions.

The determination of whether a sample is inside or outside of the window limits is made by comparing the

data format of the slot result written to the FIFO with the 20 bit window limits. An ADC sample is inside if

the following relation is true:

14.6 Lower Limit <= ADC SAMPLE <= 14.6 Upper Limit

Thus setting both limits to the same value, say 700.0 (0x2BC<<6 = 0xAF00), will only produce an inside

interrupt when the ADC sample is exactly 700.0 (0xAF00). Furthermore, note that if the lower limit is set to

zero (0x00000) and the upper limit is set to 0xFFFFF then all accumulated results from the ADC will be

inside the window limits and no excursion interrupts can ever by generated. In fact, in this case, the

incursion interrupt status bit will be set for every sample from any active slot with its window comparator bit

enabled. If the incursion interrupt is enabled then an interrupt will be generated for every such sample

written to the FIFO.

The window comparator limits are a shared resource and apply to all active slots which have their window

comparator bits enabled. If window limits are enabled for multiple enabled slots with different precision

modes, the window comparison function can be configured to automatically scale the 14.6 upper and lower

Table 1072: Window Comparator Lower Limit Register

Lower Limit

Table 1073: Window Comparator Upper Limit Register

Upper Limit