Design Ideas: January 19, 1995
The Analog Devices ADSP 2101 digital-signal-processing (DSP) µP has two serial ports. These serial ports require a receive-frame synchronization signal (Fig 1). Because a PC's RS-232C port cannot provide such a frame-synchronization signal, hooking a PC to a DSP µP may look difficult.
But, you can easily meet the frame-synchronization requirement by connecting the data-transmit line of the PC's serial port to the data-receive and receive-frame-synchronization inputs of the µP. You must program the DSP µP to accept the start bit on the transmit line as the receive-frame synchronization signal. Fig 2 shows the scheme for serial interfacing the DSP µP and a PC without using any extra hardware.
One caution: Because the DSP µP's serial ports do not have a transient-spike-rejection mechanism (as do standard USARTs), the DSP µP may receive corrupted data. USARTs check the input signal a half bit at a time after detecting a low level to confirm that the signal is actually a start bit. For the DSP µP, you can solve this problem with software. The PC's and DSP µP's communications routines must incorporate error-detection and -correction routines. The ZIPfile attached to EDN BBS /DI_SIG #1649 contains a complete writeup of this Design Idea along with communications routines (too long to reproduce here) in assembly language for the DSP µP and in C for PCs. (DI #1649)