The ACIA is illustrated in figure 3. I am using this ACIA because it is much easier to understand than newer serial interfaces. Once you understand how the . MC Asynchronous Communications Interface Adapter (ACIA) F8DCh CPCI Serial Interface MC Control/Status Register (R/W). Computers transfer data in two ways. Parallel. Serial. Parallel data transfers often 8 or more lines are used to transfer data to a device that is only a few feet away.

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A CRT terminal requires a two- way data link, because information from the keyboard is transmitted to the computer and information from the computer is transmitted to the screen.

6850 ACIA chip

As the data word length may be 7 or 8 bits with odd, even, or no parity bit, plus either one or two stop bits, there are a total of aca different possible formats for serial data transmission. A less obvious disadvantage is due to the character- oriented nature of the data link.

The chip provides the data formattingdiagram of the circuit is illustrated in figure 1.

This device relieves the system software of all the basic tasks involved in converting data between serial and parallel forms. Previous 1 2 The lower cloud contains the software that directly controls the serial interface itself. When both these bits are high, a break is transmitted by the transmitter data output pin. To clear SR2, the CPU must read the contents of the status register and then the contents of the data register.

This clock may be either one-sixteen- or sixty- four times the rate at which bits are received at the data input terminal. The overrun bit is cleared after reading data from the RDR or by a software reset.

If no parity is selected, then both the ACIA’s transmitter parity generator and receiver parity checker are disabled. The command CRA 6: The following notes provide sufficient details about the DUART’s registers to enable you to use it in its basic operating mode.

The vast majority of general- purpose microcomputers, except some entirely self- contained portable models, once used a serial interface to communicate with remote peripherals such as CRT terminals. Some systems employ more esoteric transmission paths such as fiber optics, or infra- red IR links.


Source file VHDL/ACIA_6850.vhd

Of course, this throws away the error- detecting facilities of the ACIA. Table 2 shows how the eight bits of the control register are grouped into four logical fields. The serial interface, that moves information from point- to- point one bit at a time, is generally preferred to the parallel interface, that is able to move a group of bits simultaneously. Note that CR7 is a composite interrupt enable bit and enables all the three forms of receiver interrupt described above.

The baud rate generator is bypassed when the device is used in the divide by 1 mode. Baud Rate Generator The DCD bit is set on aoscillator feeds a programmable baud rate generatorthat is capable of generating 1 of 7 baud rates for a single crystal.

Today, USB has largely replaced such interfaces. However, the following fragment of acoa input routine gives some idea of how the ‘s status register is used.

As each incoming bit is sampled, it is used to construct a new character. This preference is not due to the high performance of a serial data link, but to its low- cost, simplicity and ease of use.

Some sections of the ACIA are reset automatically by an internal power- on- reset circuit.

acia baud rate generator datasheet & applicatoin notes – Datasheet Archive

The software necessary to drive the ACIA in this minimal mode consists of three subroutines: The transmitter then sends the character, one bit at a time, by placing each successive bit on the line for a duration of T seconds, until all bits have been transmitted. For example, the instruction MOVE. They are included to. The format of the data word is650 if the internal baud rate generator is used as the receiver clock source. Odd or even parity may be selected by writing the appropriate code into bits CR2, CR3 and CR4 of the control register.

The axia mode results if the internal baud rate generator is selected for receiver data. Remember that these registers share the same address and that MR2A is selected automatically after MR1A has been loaded.


The transmitted data from the computer becomes the received data at the CRT terminal. The host computer has to read each character from a as it is received otherwise an overrun will occur and characters will be lost. It shows that the address of the lower- order byte is odd, and that the pairs of read- only and write- only registers are separated by two i. The chip provides thethe circuit is illustrated in figure 1.

The latter mode results if the internal baud rate generator is. Caia two items at the computer end of the data link enclosed in clouds in figure 1 represent the software components of the data link. When the transmitter wishes to send data, it first places the line in a space level i. This 68550 a perfectly logical, indeed an elegant, thing to do.

From the designer’s point of view, the ‘s hardware can be subdivided into three sections: Note again that SR7 is a composite interrupt bit because it is also set by an interrupt originating from the receiver side of the ACIA. Because the ACIA is a versatile device that can be operated in any of several different modes, the control register permits the programmer to define its operational characteristics.

It is also possible to operate the ACIA in a minimal interrupt- driven mode. The RDRF bit is cleared either by reading the data in the receiver data register or by carrying out a software reset on the control register. If they are not equal, a parity error flag is set to indicate a transmission error. Only its serial data input, RxD, and output, TxD, are connected to an external system. Moreover, the DUART’s baud- rate generator can be programmed simply by loading an appropriate value into a clock select register.

Once a parity error has been detected and the acua error status bit set, it remains set as long as the erroneous data remains in the receiver register. The ACIA is illustrated in figure 3.