Hardware Design Practice: 8051 Fixed-Point Expression Calculator

中文说明（README.zh-CN.md）

This repository is a course project for China University of Petroleum (Beijing). It implements an expression calculator on AT89C51 (8051 MCU) + LCD1602 + a 4×5 keypad input (4×4 matrix + 4 single keys). The calculator supports decimals and parentheses.

This repo includes the Keil project, a ready-to-flash HEX, the board schematic, and common ISP/download tools/drivers.

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Features

• Input (4×5)
  • P1: 4×4 matrix keypad (digits/operators/parentheses)
  • P3.0~P3.3: 4 single keys (Clear/Backspace/Decimal/Evaluate)
• LCD1602 UI
  • Line 1 shows the current expression (IN: prefix, tail view when too long)
  • Line 2 shows a hint or the result (= prefix)
• Expression evaluation
  • Supports + - * /, parentheses ( ), operator precedence (*/ > +-)
  • Supports unary plus/minus (e.g. -3+2, -(1+2))
• Fixed-point arithmetic (no float library)
  • SCALE = 10000 (internal value = real value × 10000)
  • Display: up to 4 decimal digits, trailing zeros trimmed (no padding)
• Error messages
  • Division by zero: DIV0
  • Invalid expression: ERR

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Quick demo

Type an expression and press =:

• 6.31*3.45 → 21.7695
• 1/16 → 0.0625
• 1/2 → 0.5
• (1+2)*3 → 9

Note: Up to 4 decimals are displayed and zeros are not padded (e.g. 1.2300 is shown as 1.23).

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Hardware & pin mapping (as implemented in code)

If your board wiring differs, update the macros and pin definitions in code/main.c.

LCD1602 (8-bit parallel)

• Data bus: P0 (LCD_PORT)
• Control pins:
• RS = P2.6
• RW = P2.5
• EN = P2.7

Keypad input

• Matrix keypad: P1 (low nibble drives rows, high nibble reads columns)
• Single keys: P3.0~P3.3 (active-low)

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Key mapping

4×4 matrix keypad (P1)

The layout is defined by key_map in main.c:

1   2   3   +
4   5   6   -
7   8   9   *
(   0   )   /

4 single keys (P3.0~P3.3)

• P3.0 (K1) → C: clear
• P3.1 (K2) → D: backspace
• P3.2 (K3) → .: decimal point
• P3.3 (K4) → =: evaluate

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Fixed-point arithmetic

To reduce code size and runtime overhead on 8051, this project avoids float and uses fixed-point numbers:

• Internal integer X = real_value × SCALE
• With SCALE = 10000, for example:
  • 6.31 is stored as 63100
  • 3.45 is stored as 34500

Note: When SCALE is large, naive a*b or a*SCALE may overflow 32-bit long. This project implements safer * and / in apply_op_scaled() using decomposition/remainder-based methods.

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Build & flash

Option A: Keil uVision3 (recommended)

1. Open the Keil project:
   • code/code.uv2
2. Confirm target chip settings (commonly AT89C51 / 12MHz).
3. Build to generate HEX:
   • Output is typically code/code.hex

Option B: use the prebuilt HEX

The repo already contains:

• code/code.hex

Serial ISP flashing (PZ-ISP)

tools/ contains common flashing tools and USB-UART drivers (e.g. PZ-ISP-for64位.exe, CH341/PL2303 drivers).

Typical steps:

1. Install the correct USB-UART driver (CH341 or PL2303 depending on your board).
2. Open PZ-ISP, select the COM port and baud rate.
3. Choose the HEX file (e.g. code/code.hex).
4. Put the board into ISP/download mode and click Download.

Entering ISP/download mode depends on your board (jumpers/buttons/power sequence). Please follow your board documentation.

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Project structure

.
├─ README.md
├─ README.zh-CN.md

├─ LICENSE
├─ HC6800EM3 v20原理图（电路图）.pdf
├─ 初始化安装与实验内容介绍.docx
├─ code/
│  ├─ main.c                 # 主程序（LCD、键盘扫描、表达式求值、定点运算）
│  ├─ code.uv2               # Keil uVision3 工程
│  ├─ code.hex               # 编译生成的HEX（可直接烧录）
│  ├─ STARTUP.A51            # 启动文件
│  └─ ...（OBJ/LST/中间文件）
└─ tools/
   ├─ PZ-ISP-for64位.exe     # 下载工具
   ├─ CH341SER.ZIP           # CH341驱动
   ├─ PL-2303.zip            # PL2303驱动
   └─ Keil3_Full.zip         # Keil安装包（仅备份/学习用途）

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FAQ

1) LCD shows garbled characters / nothing is displayed

• Check LCD power, contrast potentiometer, and RW/RS/EN wiring.
• This project uses P0 for data and P2.5/2.6/2.7 for control. Update macros if your wiring differs.

2) Keys do not respond / wrong key mapping

• Ensure the matrix keypad is wired as: low nibble = rows (output), high nibble = columns (input) on P1.
• Single keys are active-low on P3.0~P3.3.

3) Decimal results look wrong

• This project uses fixed-point numbers with SCALE=10000.
• If you change SCALE, re-check overflow risks in multiplication/division.

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License

This project is licensed under the Apache-2.0 license. See LICENSE for details.

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Acknowledgements

• Keil C51 / uVision toolchain
• LCD1602 & 8051 reference materials / course handouts