ARC Circuit Documentation

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Table of Contents

1. Introduction
2. System Block Diagram
3. Power Management
   • Li-Po Charger (MCP73831)
   • Boost/Charge Supervisor (TPS61321)
   • Power Switch & Reset
4. Processing & Display
   • Raspberry Pi Zero 2 W
   • SPI-Driven LCD
5. Keyboard Matrix
6. I²S Audio & Amplifier
   • I²S DAC (PCM5102A)
   • Line-Level Filter & Amplifier (PAM8610)
7. GPIO & Side Controls
8. Connectors & Interfaces

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1. Introduction

The ARC schematic integrates a Raspberry Pi Zero 2 W or Orange Pi Zero 2 W core with:

• A rechargeable Li-Po power subsystem
• A 39-key QWERTY matrix keyboard
• SPI LCD display interface
• High-quality I²S audio (DAC + amplifier)
• Exposed GPIO headers and side-mounted controls

The pcb is a 4 layer design

• Stackup:
  • Front (signal)
  • Layer 1 (signal)
  • Layer 2 (Ground copper pour with Power lines)
  • Layer 3 (signal)

This document explains each functional block, key components, and interconnections.

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2. System Block Diagram

   ┌────────────────────────┐     ┌──────────────────────┐
   │   Li-Po Battery (+3.7 V) ──►│  MCP73831 Charger    │
   └────────────────────────┘     └──────────────────────┘
          │                                   │
          ▼                                   ▼
   ┌────────────────────────┐     ┌──────────────────────┐
   │  TPS61321 Boost Regulator│    │  5 V / 3.3 V Rails  │
   └────────────────────────┘     └──────────────────────┘
          │                                   │
          │                                   ├─► Raspberry Pi Zero 2 W  
          │                                   ├─► Keyboard Matrix Scan  
          ▼                                   ├─► I²S DAC  
   ┌────────────────────────┐                   └─► GPIO / Side Controls
   │   Power Switch & FET  │
   └────────────────────────┘

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3. Power Management

3.1 Li-Po Charger (MCP73831)

• U8 (MCP73831): Single-cell Li-Po charger
• D4: Reverse-battery protection
• C24, C26, C28: Input/output decoupling (2.2 µF – 10 µF)
• STAT / PROG pins: Charge-status LED & current program resistor (R19)

3.2 Boost / Charge Supervisor (TPS61321)

• U5 (TPS61321): 3.7 V Li-Po to 5 V boost converter with integrated load switch
• L1: 4.7 µH inductor for boost topology
• C6–C11: Input/output filtering
• ENA pin: Enables boost when power button is pressed

3.3 Power Switch & Reset

• Q2 (MOSFET): P-channel FET controlled by the power button
• R8 (100 kΩ) and R9 (150 kΩ): Pull-ups/pull-downs for gate control
• SW4: Power on/off switch
• JTAG / Debug: TP3 (Power­_EN) and TP4 (SYS_ON) testpoints

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4. Processing & Display

4.1 SBC

• U2: Raspberry/Orange Pi Zero 2 W module footprint
• Specs:
  • Orange Pi zero 2w: 1.5 GHz processor, 4 GB ram
  • Raspberry Pi zero 2w: 1 GHz processor, 512 Mb ram
• Os: Armbian
• Exposes:
  • CPU/GPU interfaces (SPI, I²C, UART)
  • USB-OTG port (via USB-A jack X1)
  • Mini HDMI
  • 40-pin GPIO header

4.2 SPI-Driven LCD

• LCD: 3.5″ SPI TFT display
• R1 (33 Ω): Series resistor on RESET line
• R2 (100 Ω): Backlight enable gating via transistor Q1

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5. Keyboard Matrix

• Matrix layout: 5 rows × 10 columns (using 39 keys )
• D1–D40: Steering diodes (1 N4148) per key to prevent ghosting
• SW1–SW39: Tactile switches
• MCU lines:
  • Columns driven as outputs (via GPIO expander or Pi GPIO)
  • Rows read as inputs with pull-ups

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6. I²S Audio & Amplifier

6.1 I²S DAC (PCM5102A)

• U4: PCM5102A I²S DAC

6.2 Line-Level Filter & Amplifier (PAM8610)

• U9 (PAM8610): 2×5 W Class-D amplifier
• Connected to the 3.5mm jack detect pin to switch between speaker and headphones

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7. GPIO & Side Controls

Left Header

• Pin 1

  • Net: GND
  • SBC signal: GROUND
  • Function: Common ground for add-ons

• Pin 2

  • Net: +5 V
  • SBC signal: 5 V out
  • Function: Unregulated 5 V rail

• Pin 3

  • Net: GPIO12
  • SBC signal: GPIO12
  • Function: General-purpose I/O

• Pin 4

  • Net: GPIO21
  • SBC signal: GPIO21
  • Function: General-purpose I/O

• Pin 5

  • Net: GPIO22
  • SBC signal: GPIO22
  • Function: General-purpose I/O

• Pin 6

  • Net: MOSI
  • SBC signal: SPI0 MOSI
  • Function: SPI master-out, slave-in

• Pin 7

  • Net: MISO
  • SBC signal: SPI0 MISO
  • Function: SPI master-in, slave-out

• Pin 8

  • Net: SCLK
  • SBC signal: SPI0 SCLK
  • Function: SPI clock

Right Header

• Pin 1

  • Net: GND
  • SBC signal: GROUND
  • Function: Common ground for add-ons

• Pin 2

  • Net: +3.3 V
  • SBC signal: 3.3 V out
  • Function: Unregulated 3.3 V rail

• Pin 3

  • Net: +5 V
  • SBC signal: 5 V out
  • Function: Unregulated 5 V rail

• Pin 4

  • Net: GPIO16
  • SBC signal: GPIO16
  • Function: General-purpose I/O

• Pin 5

  • Net: RXD
  • SBC signal: UART0 RX
  • Function: Serial data in

• Pin 6

  • Net: TXD
  • SBC signal: UART0 TX
  • Function: Serial data out

• Pin 7

  • Net: SDA
  • SBC signal: I²C SDA
  • Function: I²C data line

• Pin 8

  • Net: SCL
  • SBC signal: I²C SCL
  • Function: I²C clock line

• Side controls:

  • Volume Up
  • Volume Down
  • Power Button

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8. Connectors & Interfaces

• X1 – USB-A Power: 5 V USB supply
• J2 – Li-Po Battery: JST PH-2 2-pin connector
• J7 – Headphone / Line-out Jack: switchable via detect pin
• J8 – Speaker: 8 Ω speaker output