Kerala Plus Two Electronics (DHSE): Microcontrollers, Microprocessors and Embedded Basics

Microcontrollers, Microprocessors and Embedded Basics tests architecture differences, 8085 and 8051 features, memory addressing, interrupts and the idea of a dedicated embedded system. Expect pin-function, capacity and interrupt-priority questions.

Core concepts

• Microprocessor vs microcontroller: a microprocessor is a CPU that needs external memory and I/O; a microcontroller integrates CPU, RAM, ROM, timers, serial port and I/O on one chip.
• 8085: 8-bit data bus, 16-bit address bus giving 2^16 = 64 KB of addressable memory, 40-pin package. AD0–AD7 are multiplexed; ALE latches the low-order address. Registers include A, B, C, D, E, H, L, flags, SP and PC. Hardware interrupts are TRAP (highest priority), RST 7.5, RST 6.5, RST 5.5 and INTR.
• 8051: 8-bit microcontroller with 4 KB on-chip ROM, 128 bytes RAM, four 8-bit ports, two timers/counters, five interrupt sources and a full-duplex UART. One machine cycle takes 12 clock cycles; with a 12 MHz crystal, that is 1 µs.
• Addressing modes and instructions: MOV A,#55H is immediate; MOVX accesses external memory. Port 0 is open-drain and needs external pull-up resistors when used as general I/O.
• Embedded system: dedicated function, real-time constraints, sensors and actuators, watchdog timer, and a choice between polling and interrupt-driven operation.

Worked example

An 8085 has a 16-bit address bus and an 8-bit data bus. Each address selects one byte, so the memory capacity is 2^16 = 65,536 bytes = 64 KB. If ALE were not used to latch the low address byte, the address would disappear from AD0–AD7 during the later part of the machine cycle and external memory would receive an invalid address.

Traps that cost marks

• Confusing address-bus width with data-bus width; 16 address lines do not mean 16-bit data.
• Placing TRAP below RST 7.5 in priority; TRAP is highest.
• Using 12 clock cycles as 12 µs at 12 MHz instead of 1 µs per machine cycle.
• Forgetting that 8051 Port 0 needs pull-ups for general I/O.
• Treating the program counter as a general-purpose register.
• Mixing up internal MOV with external MOVX.