Service manual

Page 14
Control: Controller, Front Panel and VFD Boards
The main Controller board is the heart of the Sienna. An Atmel Mega644P microcontroller (U9)
running at a clock frequency of 16MHz provides the main control functions, and a second
Mega644P (U30) is used for Keyer, VOX/AntiVOX detection, microphone sampling in FM,
keypad detection and meter backlight functions. A +/-1 PPM Temperature Compensated Crys-
tal Oscillator (TCXO), six Direct Digital Synthesis (DDS) chips and associated bandpass filters
and high bandwidth buffer/amplifiers provide a clean source of high purity, low phase noise
local oscillators for the transmitter and receiver.
Refer to the Controller schematic pages, Sheets 1 through 11. While this four layer board is
very dense and may look formidable, the circuitry is actually very straightforward. Sheet 1
shows the DC input conditioning. Two 3.3V regulators are used to drop the 5V down to 3.3V
for the DDS chips and the buffers that drive their data and address busses. R2/C3 and R38/C46
provide decoupling for the analog 5V supply that is used for the A/D converter circuits on the
microprocessors.
Main microprocessor
Sheet 2 shows the main microprocessor. This processor has 64K bytes of internal program stor-
age, 4K bytes of RAM and 2K bytes of Electrically Erasable Programmable Read-Only Mem-
ory (EEPROM). The EEPROM stores constants and lookup tables such as the filter parameters,
current state of the instrument, band plans and so on. A software timer writes data to the
EEPROM every 10 seconds if anything that needs to be saved has changed. Main memory and
EEPROM memory can be rewritten through the SPI bus (J6), which is what is used at the fac-
tory to initially program the processor, or through the RS-232C port (pins 9 and 10 on the proc-
essor, going to U7 on Sheet 4. The ST207E converts CMOS voltages into RS-232C levels
which are fed to the PC or the back panel via J4. Other RS-232C control lines (RTS, CTS) are
also converted to CMOS levels by U7 and fed to the secondary microprocessor to allow hand-
shake control and use of RTS as a CW key. RS-232C signal DTR is fed into the PTT circuitry
via Q2 and Q4. Q21 allows this signal to be disabled (via control bit SERDIS), since at boot-up,
the PC can pull the DTR line and we do not want it causing the rig to go into transmit mode!
U8 is a power-on reset chip providing a long reset pulse to start the processor correctly when
power is first applied.
LTxEn, HRcvEn
The signal that turns on (enables) the transmitter is called LTxEn. As with many signals, the
“L” means it is low true (0 Volts). (“/” is also used to indicate low true signals.) This signal is
generated by the main microprocessor in response to detection of a Push-To-Talk (/PTT) signal
from the microphone, or by the Keyer microprocessor (PTT_Keyer), which is responsible for
VOX detection and for CW keying, or by the DTR signal on the RS-232 port. DTR can be dis-
abled via an output port bit called SERDIS to keep RS-232 problems from holding the transmit-
ter on. These three signals are wire-or’d together and fed into an input port as HPTT so the
main microprocessor can tell when a transmitter enable signal is either being requested by the