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Circuit Description

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:

1. all signals of the 8251 bus-interface are initialized to their inactive values.

2. a 0-1-0 pulse is generated on the RESET signal to initialize the 8251 chip to a well-defined state.

3. a value of 0xcd is written into the 8251 mode register, which selects asynchronous communication mode with eight databits, no parity, and two stopbits - or 8N2 for short. Remember that the first command write operation following a reset is interpreted as a write operation to the mode register. Also, please check the typical system bus write sequence: first, the nCS chip-select signal is asserted (active low), then the 8-bit value 0xcd is applied to the data-bus, CnD is asserted (high) to select a command transfer, and finally a short 1-0-1 pulse is generated on the nWR write-enable input.

4. a value of 0x11 is written into the 8251 command register. Again, CnD is kept high, nCS is asserted low, the value 0x05 is applied to the databus, and a 1-0-1 pulse on the nWR signal executes the write operation. The value 0x11 activates the transmitter and disables the on-chip receiver. Also, the parity/frame/overrun error flags in the status register are reset.

5. it follows a read operation to check the 8251 status register. To avoid a short-circuit condition on the data bus, the stimuli generator first tristates the bus. Again, nCS is asserted low to activate the chip and CnD is kept high to select a status register read operation. Note that the 8251 drives the data-bus while the nRD signal is kept low. The returned value of 0x84 shows that the nDSR input is kept low (bit D7 set), while the transmitter is empty (bit D2 set) and both receiver and transmitter are ready (bit D1 and D0 cleared).

6. A value of 0x55 is written to the transmit buffer register of the 8251 to initiate a first data transfer. This time, the CnD signal is kept low to select a data transfer while nCS is kept low to select the 8251 chip. Again, a 1-0-1 pulse on the nWR signal completes the write cycle. Because the nCTS signal is asserted (low), the transmitter begins transmitting the startbit on the next falling edge of the nTXC transmitter clock. The eight databits and the two stopbits are generated and transmitted during the following ten cycles of the nTXC signal.

   Please check the waveforms to verify that the TXE (transmitter empty) output signal is asserted once the transmitter begins the actual transmission with the startbit. However, the TXRDY (transmitter ready) signal is asserted much later after the second stopbit has been transmitted.

7. A value of 0xAA is now written to the transmit buffer to initiate another data transfer.

8. A value of 0x33 is now written to the transmit buffer to initiate another data transfer. The bit sequence of those two write operations is chosen to make the structure of the startbit-databits-stopbit sequence explicit.

9. The nCTS signal is now de-asserted (high), and another value of 0x55 is written to the data register. Note that the transmission is blocked because nCTS (clear-to-send) is de-asserted. However, once nCTS goes low, the transmission starts on the next falling edge of the nTXC clock input.

10. A short phase of inactivity is introduced, to make the following group of transmissions easy to identify in the waveforms.

11. The string "\nHello, world\n" is transmitted via the USART:

           0x0a 0x0d 0x48 0x65 0x6c 0x6c 0x6f 0x2c 0x20 ...
         

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.