AM335x ADC

AM335x 有 8-channel ADC，可支援 4/5/8-線電阻式觸控。

• 8 類比輸入 AN0~AN7 (≤1.8V)，VDDA_ADC (1.8V)，VSSA_ADC (GND)，VREFN，VREFP
• ADC 是 12-Bit 的 Successive Approximation Register (SAR) ADC
• ADC 時脈最快 24MHz。
• 每個 ADC 轉換至少需要 15 ADC 時脈，包括 1x Sample Delay (SOC)、1x 取樣、12x Digitize、和 1x EOC。
• 取樣率？200K Samples per Second (24M = 15 x 8 x 200K ？)
• 輸入選自 8 類比輸入的多工。
• 整體運作是由一些 sequencer 達成，包括平常沒事在 idle，和 16 個 ADC 步驟、佮一個 touchscreen 充電步驟。
• 每個步驟 (包括 idle) 可設定個別 AFE 訊號，包括 single-ended 或 differential、RFM/INP/INM/RFP 選擇、WPN/YPN/XNP/YNN/YPP/XNN/XPP 狀態。
• 每個 ADC 步驟和充電步驟各自可以設定是否啟動，和 Open Delay。
  
• 每個 ADC 步驟可轉換一個 channel，可設定 Sample Delay、是否範圍檢查、使用 FIFO 0 或 FIFO 1、是否 Average 1/2/4/8/16、SW 或 HW、和 One-shot 或 Continuous。
  
• Programmable FSM sequencer。
• Software register bit for start of conversion
• Optional start of conversion HW synchronized to Pen touch or external HW event (but not both)
• Single conversion (one-shot) 或 Continuous conversions
• Sequence through all input channels based on a mask
• Programmable OpenDelay before sampling each channel
• Programmable sampling delay for each channel
• Programmable averaging of input samples - 16/8/4/2/1
• Differential or singled ended mode setting for each channel
• Store data in either of two FIFO groups
• Option to encode channel number with data
• Support for servicing FIFOs via DMA or CPU
• Programmable DMA Request event (for each FIFO)
• Dynamically enable or disable channel inputs during operation
• Stop bit to end conversion
• Support for the following interrupts and status, with masking:
• Interrupt for HW pen (touch) event
• Interrupt for HW Pen Up event
• Interrupt after a sequence of conversions (all non-masked channels)
• Interrupt for FIFO threshold levels
• Interrupt if sampled data is out of a programmable range
• Interrupt for FIFO overflow and underflow conditions
• Status bit to indicate if ADC is busy converting

參考

1. AM335x Sitara™ Processors datasheet
2. AM335x and AMIC110 Sitara™ Processors Technical Reference Manual

• https://www.teachmemicro.com/beaglebone-black-adc/
• http://beaglebone.cameon.net/home/reading-the-analog-inputs-adc
• https://beagleboard.org/static/librobotcontrol/group___a_d_c.html：Robotics Cape 和 BeagleBone Blue 包含兩個 voltage divider 量測 2-cell lithium battery 和 9-18V DC power jack 電壓，分別用 c_adc_batt() 和 rc_adc_dc_jack() 讀取。Robotics Cape 還有 6-pin JST-SH 插座可連結 4 potentiometers 或類比訊號到 AIN。