GR716-DS-UM, May 2019, Version 1.29
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GR716
ant the user should choose to oversample by a factor of 2^N. When a factor or 2^N is chosen and
truncation error is less important the user can shift the accumulated result by N steps.
•
Sequence sampling
for autonomously collecting of data from single or multiple ADC input
channels. The Digital ADC logic interface supports up to 4 contiguous samples. The Sequence
sampler can be combined with the oversampler i.e. samples can be optionally accumulated or
sampled with configurable distance between samples.
Triggers
can set and used for sampling. When a trigger is selected and a trigger event occur the chan-
nels value will be recorded and stored and presented in the status register. Multiple event can be
selected per channel.
Autonomously
Level detection
of internal and external voltage levels can enabled. The digital ADC
will generate an interrupt to the processor if interrupts are enabled and measured voltage level is
above or below configured thresholds. The level detection needs to be enabled in order to generate an
event. In order to get the level detection working the ADC interface needs to be configured to sample
the input. For autonomously level detection the ADC interface should be configured to sample the
input periodically.
Amplifier Control
can amplify the ADC input channel x1, x2 or x4. Amplifier control does only take
affect if the ADC input channel is configured as differential input.
On-chip
Temperature and Core Supply Voltage
can be sampled and presented to the system.
Interrupts generation
when ADC channel voltage level is outside specified voltage range or pro-
grammed sampling sequence is finished is supported.
For
low noise measurements
of external sources and DMA transfers to
offload the processor
inter-
rupts needs to be setup and used by the system. To perform a low noise measurement on a ADC chan-
nel most of the internal clock network needs to be disabled and the IOs next to the ADC inputs pins
needs to be silent. The system software needs to program the ADC to trigger and sample an input
using an external or internal trigger while system is in sleep mode. The same trigger or timer can be
used to wake the processor if needed. DMA transfers can be triggered by the interrupt generated by
the ADC interface. Simple or multiple transfers of ADC samples to internal or external RAM can be
setup and made by the DMA controller in the LEON3FT microcontroller.
Arbitration
: The ADC control units can access the ADC channels through a multiplexing structure.
When an ADC controller unit tries to start a conversion, it waits until the multiplexing structure grants
access to the ADC. ADC control units with lower IDs have higher priority, although higher priority
requests don’t stop the ongoing conversion.
12.2.3 Increasing the resolution of an ADC measurement
Applications measuring a large dynamic range, yet require fine resolution to measure small changes
in a parameter. E.g. the on-chip ADC measure a large temperature range where the system requires
the microcontroller to respond to small temperature change. Such a system could require an ADC
measurement resolution of 12-15 bits. Via oversampling and averaging, a 12-15 bit resolution mea-
surement can be supported with the on-chip 11 bit ADC.
To increase the effective number of bits (ENOB) by 1, the signal must be oversampled by a factor of
four. To support an ADC resolution of 12-15 bits the on-chip ADC needs to be configured to oversam-
ple the signal by 4, 16, 64 or 256, respectively.
Assume a system is using the on-chip 11-bit ADC to output a temperature value once every second (1
Hz). To increase the resolution of the measurement to 14-bits, we calculate the oversampling fre-
quency as follows: Sample Frequency = 4^(14-11)x1 Hz = 64Hz.
Thus, if we oversample the temperature sensor at 64 Hz, we will collect enough samples within the
required sampling period to average them and can now use 14-bits of the output data for a 14-bit mea-
surement. To do so, we accumulate i.e. add 64 consecutive samples together. Once the results have
