TM8100/TM8200 Service Manual Circuit Descriptions 97
© Tait Electronics Limited June 2006
Serial Peripheral
Interface
The control-head board uses an SPI (serial peripheral interface) to control
the display on the LCD, turn back-lighting on and off, control the
STATUS
LEDs, and read the status of the keys. The interface consists of the following
four lines at the control-head connector:
â– pin 11: data out CH SPI DO
â– pin 12: latch line CH LE
â– pin 15: data in CH SPI DI
â– pin 16: clock CH SPI CLK.
Data Input and
Output
Data that are input to the control head are clocked through the LCD driver
and daisy-chained to a shift register. Once all the data have been clocked in,
the latch line is driven low. On this falling edge all the outputs (LEDs and
LCD segments) are driven to their new state. When the latch line is driven
high, the state of each key is latched into another shift register. The data are
then clocked out back to the radio body so that the radio can respond
accordingly.
LCD Driver An oscillator is used to run the LCD. It oscillates at about 60Hz and employs
a Schmitt trigger and D flip-flop to ensure a 50% duty cycle to the LCD.
A reset circuit is required because the reset from the main board is not routed
to the control head. The reset circuit also employs a Schmitt trigger.
Volume Control The volume-control potentiometer is linear and passes the DC voltage signal
VOL WIP DC to the radio body. The signal is read by an ADC on the main
board, and the volume is adjusted accordingly.
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