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|USART 8251 transmitter demonstration
This applet demonstrates the transmitter block of the
USART 8251 or
universal synchronous and asynchronous receiver and transmitter.
For an overview and register description of the USART chip,
please visit the 8251 overview applet page.
As you can see, the circuit shown in the applet uses a single 8251 chip, with its TXD data output connected to the RX receiver input of a serial terminal. Therefore, characters transmitted by the 8251 will be decoded and displayed by the terminal, as long as the communication settings of the transmitter and the terminal match. The TXC inputs of the 8251 is driven by a clock generator which is set to a clock period of 1.6667 msec or 600 baud. A stimuli generator component is used to automatically initialize the USART for transmission and to demonstrate the timing of the transmitter and its status signals. The communication parameters of both the 8251 and the terminal are set to eight databits, no parity, and two stopbits.
Once the simulation starts, the stimuli generator component applies the following sequence of input changes to the circuit:
When the automatic sequence has finished, you should take a look at the signal waveforms to check and understand the RS-232 datastream and the values returned by the status register read operations. The waveform of the extra comment signal includes a short description of the ongoing operation. Just click the repaint button in the waveform viewer to update the waveforms, and use the zoom buttons or the zoom-region option to enlarge the areas of interest. For easy comparison between data bus values and transmitter waveforms, you might want to select hexadecimal number format in the waveform viewer (via menu, options, number format, hex). The screenshot below shows example waveforms during the first block of transmitted characters.
You can also click on the symbol of the serial terminal component, to open (or close) the user-interface window of the terminal and check that the transmitted data characters were indeed received correctly.
To further explore the circuit, just continue the simulation via clicking the "run" button (play) in the simulator control panel. You can now use the DATA input switch and the nWR write-enable switch to transmit other data characters via the USART chip. You can also type the following bindkeys to control the applet: 'r' for nRD, 'w' for nWR, 'x' for RESET.
|Print version | Run this demo in the Hades editor (via Java WebStart)|
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