Modes Of Operation - Texas Instruments VCA8613EVM User Manual

Microcontroller
The CW outputs need a compliance voltage of +3V. The circuit in Figure 1
shows how the compliance voltage is applied to the VCA861x device through
a 220µH inductor. Extra provisions have been made to this circuit for future
VCA devices.
As configured from the factory, the CW output circuit allows the noise of the
CW path to be measured. This configuration is the reason for the large feed-
back resistor in the output amplifier circuit. However, when trying to sum more
than two channels to one CW output, this resistor may pose a problem. Before
summing more than two CW channels, this feedback resistor should be re-
duced in value. For help in determining the minimum value of resistor needed,
refer to the VCA8613 data sheet for more information about the maximum cur-
rent draw for the CW outputs.
4 Microcontroller
The VCA8613EVM contains a microcontroller to access functions on the
VCA8613. The microcontroller provides the evaluation module the ability to run
in a stand-alone mode or be controlled by software running on a PC. The micro-
controller is run from its own internal oscillator set to 12MHz. The only external
clock signals are those used to communicate to the VCA8613 via the serial data
clock line (for clocking data to the VCA8613), and the embedded clock signal in
the RS-232 communications circuit. The RS-232 clock operates only when data
is being sent from the PC to the microcontroller. The serial communication from
the PC to the microcontroller is set to 19200kbps. This baud rate is set internally
on the microcontroller and is configured automatically in the PC software.
5

Modes of Operation

There are three modes of operation for the VCA8613EVM: Stand-Alone, PC-
Control, and Direct-Control. The first two modes are accessed by using the S1
switches and the S2 pushbutton. S2 is used as the reset button to make pro-
gram changes on the microcontroller. Table 1 shows the different modes of op-
eration.
Table 1. VCA8613EVM Modes of Operation
SW3
4
SW2 SW1 Mode of Operation
0 0 0 Power Down
0 0 1 TGC Mode 1: Attenuator = 29dB, PGA gain = 21dB
0 1 0 TGC Mode 2: Attenuator = 29dB, PGA gain = 26dB
0 1 1 TGC Mode 3: Attenuator = 40dB, PGA gain = 26dB
1 0 0 CW Mode 1
1 0 1 CW Mode 2
1 1 0 CW Mode 3
1 1 1 PC-Control Mode or Direct-Control Mode