8251A Control Words

8251A Control Words

AU : Dec.-07

The control words defines the complete functional definition of 8251A and they must be loaded before any transmission or reception. The control words of 8251A are split into two formats :

1. Mode instruction 2. Command instruction

Mode Instruction : Fig. 11.5.1 shows the mode instruction format.

The instruction can be considered as four 2-bit fields. The first 2-bit field (D₁-D₀) determines whether the USART is to operate in the synchronous (00) or asynchronous mode. In the asynchronous mode, this field determines the division factor for clock to decide the baud rate. For example, if D₁ and D₀ are both ones, the   will be divided by 64 to establish the baud rate.

The second 2-bit field (D₃-D₂) determines number of data bits in one character. With this 2-bit field we can set character length from 5-bits to 8 bits.

The third 2-bit field, (D₅-D₄), controls the parity generation. The parity bit is added to the data bits only if parity is enabled.

The last field, (D₇-D₆), has two meanings depending on whether operation is to be in the synchronous or asynchronous mode. For asynchronous mode, (i.e. D₁D₀ ≠ 00), it controls the number of STOP bits to be transmitted with the character. In synchronous mode, (i.e. D₁D₀ = 00) this field controls the synchronizing process. It decides whether to operate with external synchronization or internal synchronization and whether to transmit single synchronizing character or two synchronizing characters.

Command Instruction : After the mode instruction, command character should be issued to the USART. It controls the operation of the USART within the basic frame work established by the mode instruction. Fig. 11.5.2 shows command instruction format.

It does function such as : Enable Transmit/Receive, Error Reset and Modem Control.