Data Transfer from ADC to Ping-Pong Buffers

The ADC is auto-triggered from a sampling timer. See Analog-to-Digital Conversion for details about ADC sampling and acquisition. The 12-bit conversion result from the ADC is available in the ADRESH:L registers. The ADC result can be left-aligned so that 8-bit values stored in ADRESH register can be transferred directly to the ping-pong buffers using DMA⁽¹⁾. The DMA setup configurations are listed in Table 1.

Table 1. DMA Setup Configuration (DMA2)
DMA2 Setup
SSA	Source Address	&ADRESH
SSZ	Source Size	1
SMR	Source Memory Region	SFR
SMODE	Source Address Mode	SPTR remains unchanged
DSA	Destination Address	Beginning of ping buffer
DSZ	Destination Size	512 (2 banks for ping and pong buffers)
DMODE	Destination Address Mode	DPTR is incremented
SIRQEN	Start Trigger Enable	Yes
SIRQ	Start Trigger	AD (ADC conversion)
SSTP	Source Counter Reload Stop	No - SIRQEN not cleared
DSTP	Destination Counter Reload Stop	No - SIRQEN not cleared
AIRQEN	Abort Trigger Enable	No (stops when SCNT reloads)
AIRQ	Abort Trigger	N/A

How It Works:

1.The DMA is triggered as soon as an ADC conversion is complete (ADIF trigger) and the content of the ADRESH register is transferred to the first address in the ping buffer. DPTR is incremented and now points to the next address in the ping buffer. Because SSZ = 1, the DMA stops after the first transfer and waits for the next trigger.
2.After 256 such iterations, the ping buffer is full and a timer interrupt sends a signal to the software to initiate data transfer from the ping buffer to memory. Meanwhile, DPTR increments and enters the pong buffer range⁽²⁾.
3.For the next few iterations, while the pong buffer is filling up, the software/hardware is busy transferring data out of the ping buffer.
4.After another 256 iterations (once the pong buffer is full), another timer interrupt sends another signal to the software to transfer data from the pong buffer to memory. Meanwhile DPTR reaches the end of the allocated buffer size and rolls back to the starting DSA address, which is the beginning of the ping buffer⁽²⁾.
5.This process continues as long as the recording is in progress.

Important:

1.While most of the newer PIC18 devices have a 10-bit or 12-bit ADC, only 8-bit ADC values are used to keep the application simple. This is because the DAC used to playback the audio signal has a resolution of 8-bits.
2.In this application, the ping and pong buffers are adjacent to each other in data memory. If ping and pong buffers are separated in the data memory, then SSA needs to be reconfigured after every 256 iterations.
3.The DMA servicing the ADC has the highest priority among all the DMAs used. This is to avoid the other DMAs from blocking the sampling and storage of new audio samples.