Page 224 of 229 Start Back Next End ZBasic Language Reference 214 ZBasic Microcontrollers Clock Source Serial Interface In many cases, you’ll want to provide serial interface circuitry to convert the AVR serial channel signals to/from RS-232 levels. Note that not all AVR chips have on-board serial hardware (UART) but if it does and you elect to install the ZBasic bootloader (or other bootloader) you’ll probably need the serial interface circuitry for serial channel Com1. The recommended serial interface circuitry is shown below. This circuit has two sub components. The MAX232 chip functions as an RS-232 level converter that translates the 0-Vcc signals of the AVR to/from the standard RS-232 voltage levels. The second component is the ATN circuitry that is used to signal the AVR to go into download mode when using the ZBasic bootloader. The ATN circuitry is that portion connected between pin 4 of the DB-9 serial connector and the /RST pin of the AVR. If you do not intend to use the ZBasic bootloader, the ATN circuitry can be omitted. You may want to include a jumper in the ATN circuitry so that it can be disconnected when downloads are not required. With the ATN circuitry connected, the AVR will receive a reset pulse on every positive transition of pin 4 (DTR) of the serial connector. If alternate component values are chosen you must ensure that the AVR receives a reset pulse of at least 2uS on every positive transition of the DTR signal on pin 4 of the serial connector. One advantage to using this recommended circuit is that the MAX232 chip includes a DC-DC converter that produces nominally +/- 10V from the positive Vcc source. This is particularly useful if your application requires one or more of these voltages. For example, some LCD devices require a low-current negative supply for their backlight circuitry. The positive voltage is available on pin 2 of the MAX232 while the negative voltage is available on pin 6. Consult the datasheet for the MAX232 device for information on the current capacity of these supplies. Also note that the MAX232 may not operate at the voltage level that you’ve chosen for your AVR. There are alternate RS-232 level converter chips that operate at different or wider voltage ranges. For example, the MAX3232E can operate from 3.3V to 5V.