Adding documentation for ADCs and DACs

src/adc.rs

@@ -1,18 +1,75 @@
 ///! Stabilizer ADC management interface
 ///!
-///! The Stabilizer ADCs utilize a DMA channel to trigger sampling. The SPI streams are configured
+///! # Design
-///! for full-duplex operation, but only RX is connected to physical pins. A timer channel is
-///! configured to generate a DMA write into the SPI TXFIFO, which initiates a SPI transfer and
-///! results in an ADC sample read for both channels.
 ///!
-///! In order to read multiple samples without interrupting the CPU, a separate DMA transfer is
+///! Stabilizer ADCs are connected to the MCU via a simplex, SPI-compatible interface. The ADCs
-///! configured to read from each of the ADC SPI RX FIFOs. Due to the design of the SPI peripheral,
+///! require a setup conversion time after asserting the CSn (convert) signal to generate the ADC
-///! these DMA transfers stall when no data is available in the FIFO. Thus, the DMA transfer only
+///! code from the sampled level. Once the setup time has elapsed, the ADC data is clocked out of
-///! completes after all samples have been read. When this occurs, a CPU interrupt is generated so
+///! MISO. The internal setup time is managed by the SPI peripheral via a CSn setup time parameter
-///! that software can process the acquired samples from both ADCs. Only one of the ADC DMA streams
+///! during SPI configuration, which allows offloading the management of the setup time to hardware.
-///! is configured to generate an interrupt to handle both transfers, so it is necessary to ensure
+///!
-///! both transfers are completed before reading the data. This is usually not significant for
+///! Because of the SPI-compatibility of the ADCs, a single SPI peripheral + DMA is used to automate
-///! busy-waiting because the transfers should complete at approximately the same time.
+///! the collection of multiple ADC samples without requiring processing by the CPU, which reduces
+///! overhead and provides the CPU with more time for processing-intensive tasks, like DSP.
+///!
+///! The automation of sample collection utilizes two DMA streams, the SPI peripheral, and a timer
+///! compare channel for each ADC. The timer comparison channel is configured to generate a
+///! comparison event every time the timer is equal to a specific value. Each comparison then
+///! generates a DMA transfer event to write into the SPI TX buffer. Although the SPI is a simplex,
+///! RX-only interface, it is configured in full-duplex mode and the TX pin is left disconnected.
+///! This allows the SPI interface to periodically read a single word whenever a word is written to
+///! the TX side. Thus, by running a continuous DMA transfer to periodically write a value into the
+///! TX FIFO, we can schedule the regular collection of ADC samples in the SPI RX buffer.
+///!
+///! In order to collect the acquired ADC samples into a RAM buffer, a second DMA transfer is
+///! configured to read from the SPI RX FIFO into RAM. The request for this transfer is connected to
+///! the SPI RX data signal, so the SPI peripheral will request to move data into RAM whenever it is
+///! available. When enough samples have been collected, a transfer-complete interrupt is generated
+///! and the ADC samples are available for processing.
+///!
+///! The SPI peripheral internally has an 8- or 16-byte TX and RX FIFO, which corresponds to a 4- or
+///! 8-sample buffer for incoming ADC samples. During the handling of the DMA transfer completion,
+///! there is a small window where buffers are swapped over where it's possible that a sample could
+///! be lost. In order to avoid this, the SPI RX FIFO is effectively used as a "sample overflow"
+///! region and can buffer a number of samples until the next DMA transfer is configured. If a DMA
+///! transfer is still not set in time, the SPI peripheral will generate an input-overrun interrupt.
+///! This interrupt then serves as a means of detecting if samples have been lost, which will occur
+///! whenever data processing takes longer than the collection period.
+///!
+///!
+///! ## Starting Data Collection
+///!
+///! Because the DMA data collection is automated via timer count comparisons and DMA transfers, the
+///! ADCs can be initialized and configured, but will not begin sampling the external ADCs until the
+///! sampling timer is enabled. As such, the sampling timer should be enabled after all
+///! initialization has completed and immediately before the embedded processing loop begins.
+///!
+///!
+///! ## Batch Sizing
+///!
+///! The ADCs collect a group of N samples, which is referred to as a batch. The size of the batch
+///! is configured by the user at compile-time to allow for a custom-tailored implementation. Larger
+///! batch sizes generally provide for more processing time per sample, but come at the expense of
+///! increased input -> output latency.
+///!
+///!
+///! # Note
+///!
+///! While there are two ADCs, only a single ADC is configured to generate transfer-complete
+///! interrupts. This is done because it is assumed that the ADCs will always be sampled
+///! simultaneously. If only a single ADC is used, it must always be ADC0, as ADC1 will not generate
+///! transfer-complete interrupts.
+///!
+///! There is a very small amount of latency between sampling of ADCs due to bus matrix priority. As
+///! such, one of the ADCs will be sampled marginally earlier before the other because the DMA
+///! requests are generated simultaneously. This can be avoided by providing a known offset to the
+///! sample DMA requests, which can be completed by setting e.g. ADC0's comparison to a counter
+///! value of 0 and ADC1's comparison to a counter value of 1.
+///!
+///! In this implementation, single buffer mode DMA transfers are used because the SPI RX FIFO can
+///! be used as a means to both detect and buffer ADC samples during the buffer swap-over. Because
+///! of this, double-buffered mode does not offer any advantages over single-buffered mode (unless
+///! double-buffered mode offers less overhead when accessing data).
 use super::{
     hal, sampling_timer, DMAReq, DmaConfig, MemoryToPeripheral,
     PeripheralToMemory, Priority, TargetAddress, Transfer, SAMPLE_BUFFER_SIZE,

src/dac.rs

@@ -1,8 +1,45 @@
 ///! Stabilizer DAC management interface
 ///!
-///! The Stabilizer DAC utilize a DMA channel to generate output updates.  A timer channel is
+///! # Design
-///! configured to generate a DMA write into the SPI TXFIFO, which initiates a SPI transfer and
+///!
-///! results in DAC update for both channels.
+///! Stabilizer DACs are connected to the MCU via a simplex, SPI-compatible interface. Each DAC
+///! accepts a 16-bit output code.
+///!
+///! In order to maximize CPU processing time, the DAC code updates are offloaded to hardware using
+///! a timer compare channel, DMA stream, and the DAC SPI interface.
+///!
+///! The timer comparison channel is configured to generate a DMA request whenever the comparison
+///! occurs. Thus, whenever a comparison happens, a single DAC code can be written to the output. By
+///! configuring a DMA stream for a number of successive DAC codes, hardware can regularly update
+///! the DAC without requiring the CPU.
+///!
+///! ## Multiple Samples to Single DAC Codes
+///!
+///! For some applications, it may be desirable to generate a single DAC code from multiple ADC
+///! samples. In order to maintain timing characteristics between ADC samples and DAC code outputs,
+///! applications are required to generate one DAC code for each ADC sample. To accomodate mapping
+///! multiple inputs to a single output, the output code can be repeated a number of times in the
+///! output buffer corresponding with the number of input samples that were used to generate it.
+///!
+///!
+///! # Note
+///!
+///! There is a very small amount of latency between updating the two DACs due to bus matrix
+///! priority. As such, one of the DACs will be updated marginally earlier before the other because
+///! the DMA requests are generated simultaneously. This can be avoided by providing a known offset
+///! to other DMA requests, which can be completed by setting e.g. DAC0's comparison to a
+///! counter value of 2 and DAC1's comparison to a counter value of 3. This will have the effect of
+///! generating the DAC updates with a known latency of 1 timer tick to each other and prevent the
+///! DMAs from racing for the bus. As implemented, the DMA channels utilize natural priority of the
+///! DMA channels to arbitrate which transfer occurs first.
+///!
+///!
+///! # Future Improvements
+///!
+///! In this implementation, single buffer mode DMA transfers are used. As a result of this, it's
+///! possible that a timer comparison could be missed during the swap-over, which will result in a
+///! delay of a single output code. In the future, this can be remedied by utilize double-buffer
+///! mode for the DMA transfers.
 use super::{
     hal, sampling_timer, DMAReq, DmaConfig, MemoryToPeripheral, TargetAddress,
     Transfer, SAMPLE_BUFFER_SIZE,
@@ -90,12 +127,11 @@ macro_rules! dac_output {
                 let mut spi = spi.disable();
                 spi.listen(hal::spi::Event::Error);
 
-                // Allow the SPI FIFOs to operate using only DMA data channels.
+                // AXISRAM is uninitialized. As such, we manually zero-initialize it here before
-                spi.enable_dma_tx();
+                // starting the transfer.
-
+                for byte in DAC_BUF[$index].iter_mut() {
-                // Enable SPI and start it in infinite transaction mode.
+                    *byte = 0;
-                spi.inner().cr1.modify(|_, w| w.spe().set_bit());
+                }
-                spi.inner().cr1.modify(|_, w| w.cstart().started());
 
                 // Construct the trigger stream to write from memory to the peripheral.
                 let transfer: Transfer<_, _, MemoryToPeripheral, _> =
@@ -108,6 +144,15 @@ macro_rules! dac_output {
                         trigger_config,
                     );
 
+                transfer.start(|spi| {
+                    // Allow the SPI FIFOs to operate using only DMA data channels.
+                    spi.enable_dma_tx();
+
+                    // Enable SPI and start it in infinite transaction mode.
+                    spi.inner().cr1.modify(|_, w| w.spe().set_bit());
+                    spi.inner().cr1.modify(|_, w| w.cstart().started());
+                });
+
                 Self {
                     transfer,
                     // Note(unsafe): This buffer is only used once and provided for the next DMA transfer.
@@ -131,15 +176,9 @@ macro_rules! dac_output {
                 &mut self,
                 next_buffer: &'static mut [u16; SAMPLE_BUFFER_SIZE],
             ) {
-                // If the last transfer was not complete, we didn't write all our previous DAC codes.
-                // Wait for all the DAC codes to get written as well.
-                if self.first_transfer {
-                    self.first_transfer = false
-                } else {
                 // Note: If a device hangs up, check that this conditional is passing correctly, as
                 // there is no time-out checks here in the interest of execution speed.
                 while !self.transfer.get_transfer_complete_flag() {}
-                }
 
                 // Start the next transfer.
                 self.transfer.clear_interrupts();