RFXCOM RFXtrx433 Especificaciones descargar pdf (Pagina 47)

Chapter 4. Implementation 40

4.1.3. USB-serial-bridge Replacement

The USB-serial-bridge FT232RL was initially selected because it can easily be integrated into the

circuit via serial interface and relieves the ﬁrmware due to its own processing unit and prefabricated

functions. However, it is highly priced and can only operate as a virtual serial device. To reduce the

costs and increase the ﬂexibility of the USB, the ATXMega192A3 was replaced by a newer version

with USB connectivity. That is the ATXMega192A3U for the WiﬁCtrl and the ATXMegaA4U for

the MiniCtrl, in which the U in the name stands for USB. Figure 4.5 shows the old circuit with

FT232RL and the new one with integrated USB.

FT232RL

ESD

MCU

D-

D+

USB

+5V

D-

D+

USB

+5V

1 2

GND

PRTR5V0U2X

EMI

GND

Figure 4.5.: The old USB circuit (top) and the newer circuit with EMI ﬁltering and the IC

PRTR5V0U2X for ESD protection (bottom).

The FT232RL was removed and the USB data lines D+ and D- were connected directly to the

MCU. As recommended by Atmel [30], an ESD protection was integrated into the data lines

to prevent power surges which can damage the platform. In addition, the WiﬁCtrl has an EMI

protection to allow high cable length without disturbances [31]. However, the smaller MiniCtrl

has no EMI circuit due to lack of space on the two layer PCB. This is no limitation, as the device

is plugged directly to a computer and the cable length can be neglected.

The Atmel software framework ASF is used for handling the USB traﬃc as well as initializing the

communication device class (CDC) in the ﬁrmware. It creates a virtual serial port which can be

accessed through Atmel drivers on a computer. The virtual serial port allows simple control with

every terminal program and easy integration into individual software. The ﬁrmware is not limited

to CDC; the ASF also supports HID and custom USB classes which makes the platform suitable

for custom applications, where CDC is not appropriate. The USB clock is generated internally by

multiplying the processor clock by two, consequently the processor clock had to be scaled down

from 32 Mhz to 24 Mhz to match the 48 Mhz USB clock. This reduces the overall speed of the

system and therefore increases the base load, which aﬀects the real-time capabilities of the system.

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RFXCOM RFXtrx433 Especificaciones Pagina 47