Intel mcs-48 manual Features and Enhancements

8251 A/S2657

FEATURES AND ENHANCEMENTS

8251A is anadval'lced designon"-e industry stan, dard USAR:r, the Intel® 8251. The 8251A oper- ates with an extended range of Intel micropro- cessors that includes the new 8085 CPU and main- tains compatibility with the 8251. Familiarization time is minimal because of compatibility and involves only knowing the additional features and enhancements, and reviewing the AC and DC ·speci- fications of the 8251A.

The 8251A incorporates all the key features of the 8251 and has the following additional features and enhancements:

•8251A has double-buffered data paths with separate I/O registers for control, status, Data In, and Data Out, which considerably simplifies control programming and mini- mizes CPU overhead.

•In asynchronous operations, the Receiver detects and handles "break" automatically, relieving the CPU of this task.

•A refined Rx initialization prevents the Receiver from starting when in "break" state, preventing unwanted interrupts from a disconnected USART.

•At the conclusion of a transmission, TxD line will always return to the marking state unless SBRK is programmed.

•Tx Enable logic enhancement pre~ents a Tx Disable command from halting trans- mission until all data previously· written has been transmitted. The logic also prevents the transmitter from turning off in the middle of a word.

•When External Sync Detect is programmed, Internal Sync Detect is disabled, and an Ex- ternal Sync Detect status is provided via a flip-flop which clears itself upon a status read.

•Possibility of false sync detect is minimized by ensuring that if double character sync is programmed, the characters be contiguously detected and also by clearing the Rx register to all ones whenever Enter Hunt command is issued in Sync mode.

•As iong as the 825iA is not seiecteu, the RD and WR do not affect the internal opera- tion of the device.

•The 8251A Status can be read at any time but the status update will be inhibited during status read.

•The 8251A is free from extraneous glitches and has enhanced ACand DC characteristics, providing higher speed and better operating margins.

•Synchronous Baud rate from DC to 64K.

•Fully compatible with Intel's new industry standard, the MCS-85.

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