Chapter 2 Everything

Bus Systems and Devices in Embedded Computer Systems

• Many advanced embedded processors have large on-chip memories and high speeds, but still require external bus interfaces for specialized memory needs and controlling external devices.

• The Raspberry Pi (RPi) is a low-cost single-board computer with a SoC from Broadcom that can run the Linux operating system and interface with low-level electronics for IoT and other applications.

• The Broadcom SoC used in the Compute Module 3 (CM3) has a CPU which uses an ARM Cortex A core and various hardware blocks, such as a GPU, memory, timers, DMA, GPIO, USB, PCM, I2S, PWM, and serial communication.

• The on-chip bus in the Broadcom SoC uses a bus architecture designed by ARM called AMBA, which specifies interfaces to external devices like the Advanced eXtensible Interface (AXI).

• The AXI specification allows connection to external memory using the external bus interface, and for Broadcom devices, this is known as the Secondary Memory Interface (SMI).

• The 8080 or 6800 bus interface is commonly used for graphical LCDs and flash memories, and its protocol is still widely used in modern systems.

• The 8080 interface specifies a 16-bit address bus, 8-bit data bus, and separate read and write signals for control, with the IO/M signal distinguishing between addressing memory and I/O devices.

• Memory devices like RAM and ROM have different technologies and uses, with RAM being volatile and capable of storing and retrieving data quickly, while ROM is non-volatile and stores data permanently.

• EPROM is erasable and re-usable, while PROM is programmable but non-reversible, and EEPROM is electrically erasable and can be re-written in circuit.

• Flash memory is a type of EEPROM that updates its contents in groups of bytes or sectors, and is available as bare memory chips or memory cards using NAND gate technology.

• Memory cards like Compact Flash, Secure Digital, and MultiMedia Card formats are commonly used for sequential access of data within blocks of memory.

• Interfacing to memory devices involves hardware design and may require software effort, depending on the type of device and application.Bus Systems and Devices for Embedded Computer Systems

• Flash memory is used for program updates and storing large amounts of data.

• CF cards appear like hard disks while SD/MMC cards use the Serial Peripheral Interface (SPI) protocol.

• Serial EEPROM is used for storing settings and configuration information.

• RAM devices can be static or dynamic, with static RAM using flip-flops and dynamic RAM using MOS capacitors.

• I/O plug-in cards are used to provide more ports for interfacing with external devices.

• Buffers increase the driving power of microprocessors for driving other devices.

• Tri-state buffers allow signals to pass through without altering the logic value and bi-directional buffers control the direction of data flow.

• Latches hold signals at a particular logic level until a change is requested.

• Basic input ports use tri-state buffers and basic output ports use latches.

• A monitor program or BIOS is stored in ROM and allows users to perform fundamental tasks like input and output.

• The boot up process for the 8080 processor involves executing code found in ROM, searching for a secondary storage device, reading in the boot sector, and initializing the system through "bootstrapping."

• The BIOS may occupy up to 64K of ROM, but monitor programs can be as small as 2K.