AP-283 Flexibility in Frame Size with the 8044
The 8044 is a serial communication microcontroller known as the RUPI (Remote Universal Peripheral Interface). It merges the popular 8051 8-bit microcontroller with an intelligent, high performance HDLC/SDLC serial communication controller called the Serial Interface Unit (SIU). The chip provides all features of the
microcontroller and supports the Synchronous Data Link Control (SDLC) communications protocol. There are two methods of operation relating to frame size:
1) Normal operation (limited frame size)
2) Expanded operation (unlimited frame size)
In Normal operation the internal 192 byte RAM is used as the receive and transmit buffer. In this operation, the chip supports data rates up to 2.4 Mbps exter-
nally clocked and 375 Kbps self-clocked. For frame sizes greater than 192 bytes, Expanded operation is required. In Expanded operation the external RAM, in
conjunction with the internal RAM, is used as the transmit and receive buffer. In this operation, the chip supports data rates up to 500 Kbps externally clocked
and 375 Kbps self-clocked. In both cases, the SIU handles many of the data link functions in hardware, and the chip can be configured in either Auto or Flexible
mode.
The discussion that follows describes the operation of the chip and the behavior of the serial interface unit. Both Normal and Expanded operations will be further
explained with extra emphasis on Expanded operation and its supporting software. Two examples of SDLC communication systems will also be covered, where the
chip is used in Expanded operation. The discussion assumes that the reader is familiar with the 8044 data sheet and the SDLC communications protocol.
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