B&B Electronics Asynchronous to Synchronous Converter 2010 Manual do Utilizador descarregar pdf (Página 21)

Department of Computer Science and Electronics

4.2. Hardware, schematic and layout

4.2.1. Required components for one card

⇒ Atmega16 microcontroller

⇒ Max489 transceiver

⇒ 8 OPB607 infrared sensors

⇒ 2 sockets RS485

⇒ JTAG pin out

⇒ BC337 NPN transistor

⇒ Ceramic oscillator, 16MHz

⇒ 5 capacitors, 0.1μF

⇒ 14 resistors

o 1 resistor, 2.2kΩ

o 4 resistors, 120Ω

o 9 resistors, 10kΩ

⇒ Cable RS485

4.2.2. Schematic and layout

The emitters of the infrared sensors’ transistor are connected to the

microcontroller’s ADC input. At the same time the cathodes of the sensors’ diode is

connected to the collector of a NPN transistor’s pin. The purpose of this is to consume

exactly the necessary current from the sensors. The base of the transistor is connected to

the PD3 pin in the Atmega16, thus, that pin can be set just before reading the sensors’

values and be disconnected when reading is finished, saving the unnecessary current

and the 75mW that both the diode and the photo Darlington dissipated.

The reference voltage for the ADC (V

REF) in the Atmega16 indicates the

conversion range for the ADC. In this case, it is connected to Avcc, which is 5 Volts.

There are some reasons for the fact that 4 capacitors are connected in parallel.

Firstly, in order for the interferences not to enter the CI in question, since having a

digital device such a microcontroller can alter its normal functionality by changing the

value of some registers. These capacitors also avoid the bad effects that the

commutation noise produces. As the Atmega16 is requesting load of several mA in its

pins, it has to raise the voltage of all or several pins’ to the tension of the feeding

demanding a major consumption than needed. At the time of the connection, a

weakening of the tension will take place in the feeding of the microcontroller, producing

a not as fast as expected response in the output of the pins, and for this cause and for the

purpose of not creating current loops, the capacitor must always be as close to the pins

as possible.

On the other hand, the AVCC pin is the supply voltage pin for Port A and the

A/D Converter, and it should be externally connected to V

CC. [1]

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