SILICON GATE MOS 8251

Receiver Buffer

The Receiver accepts serial data, converts this serial input to parallel format, checks for bits or characters that are unique to the communication technique and sends an "assembled" character to the CPU. Serial data is input to the RxD pin.

When used as an input, (external SYNC detect mode), a positive going signal will cause the 8251 to start assembling data characters on the falling edge of the next RxC. Once in SYNC, the "high" input signal can be removed. The dura- tion of the high signal should be at least equal to the period of RxC.

Receiver Control

This functional block manages all receiver-related activities.

RxRDY (Receiver Ready)

This output indicates that the 8251 contains a character that is ready to be input to the CPU. RxRDY can be connected to the interrupt structure of the CPU or for Polled opera- tion the CPU can check the condition of RxRDY using a status read operation. RxRDY is automatically reset when the character is read by the CPU.

RxC (Receiver Clock)

The Receiver Clock controls the rate at which the character is to be received. In Synchronous Mode, the frequency of RxCisequal to the actual Baud Rate (1x). In Asynchronous Mode, the frequency of RxC is a multiple of the actual Baud Rate. A portion of the mode instruction selects the value of the multipl ier; it can be 1x, 16x or 64x the Baud Rate.

For Example:

If Baud Rate equals 300 Baud,

 

RxC equals 300 Hz (1 x)

 

RxC equals 4800 Hz (16x)

 

RxC equals 19.2 kHz (64x).

 

If Baud Rate equals 2400 Baud,

 

RxC equals 2400 Hz (1 x)

 

RxC equals 38.4 kHz (16x)

 

RxC equals 153.6 kHz (64x).

Data is sampled into the 8251 on the rising edge of RxC.

NOTE: In most communications systems, the 8251 will be handling both the transmission and reception operations of a single link. Consequently, the Receive and Transmit Baud Rates will be the same. Both TxC and RxC will require iden- tical frequencies for this operation and can be tied together and connected to a single frequency source (Baud Rate Generator) to simpl ify the interface.

SYNDET (SYNC Detect)

This pin is used in SYNChronous Mode only. It is used as either input or output, programmable through the Control Word. It is reset to "low" upon RESET. When used as an output (internal Sync mode), the SYNDET pin will go "high" to indicate that the 8251 has located the SYNC character in the Receive mode. If the 8251 is programmed to use double Sync characters (bi-sync), then SYNDET will go "high" in the middle of the last bit of the second Sync character. SYNDET is automatically reset upon a Status Read operation.

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8251 Interface to 8080 Standard System Bus

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Image 206
Intel 8080 manual Receiver Buffer, Receiver Control, RxRDY Receiver Ready, RxC Receiver Clock, Syndet Sync Detect

8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.