Keypad Side Keys And Top Keys; Microphone Preamplifier And Ptt - Motorola ASTRO W3 Service Manual

3-2
Operation of the circuit is complex; a brief description is detailed below:
• Display data exits U3-5/12 as eight bits of parallel data. This data addresses a character
look-up table in LCD driver/controller U4. This provides the relevant segment output data from
U4, as well as LCD drivers U5, U6, and U7, to activate the relevant display dots that form the
characters as defined in the look-up table.
• The LCD is multiplexed with eight backplanes that are driven separately by U4-47 to -54. Each
backplane is driven sequentially, one after the other, by a stepped a.c. voltage of between
approximately -2 volts and +5 volts. LCD dots are displayed when a maximum or minimum of
one of the backplane drives coincides with a minimum or maximum, respectively, of the
relevant segment drive voltage.
• Negative voltage generation necessary for correct operation of the display is derived from a
timed clock output from U3-38. This is fed to a voltage doubler comprising C21, C22, and D30.
The d.c. output of the doubler is fed to voltage divider R16 to R20. This divider provides evenly
spaced voltage increments from approximately -2 volts to +5 volts, which are then fed to U4,
U5, U6, and U7 to form the stepped backplane and segment drive voltages.
• Temperature compensation is provided by U3 and U14 in conjunction with D32. This diode
provides a temperature-dependent voltage to analog input pin 1 of A/D converter U14. This is
sent as serial data input to U3-34, where it is compared to a reference in U3. The amount of
deviation from this reference determines the duty cycle of the output drive voltage from U3-38.
This results in a change to the drive voltage across C22, which is fed to analog input pin 6 of
U14 via voltage divider R26 and R2, which is then fed to U3 as serial data. U3 checks this
voltage for accuracy, using the temperature-dependent voltage at U14-1 as a reference, and
corrects the drive duty cycle accordingly.
• Contrast control is achieved by adjusting the display drive duty cycle U3-38 by either keypad
control or radio programming via the SB9600 bus. U3 contains a reference that presets the
drive duty cycle to an average value when first connected to a new radio. Adjustment of the
contrast setting will set up a new reference, which will be retained by the radio in EEPROM for
future use so that the new setting will be retained if the microphone is turned off.
3.1.3

Keypad Side Keys and Top Keys

The HHCH keypad is configured as a matrix of five columns by six rows. The keyboard is scanned
by taking each column output low in turn (each one 1 ms) and reading the corresponding row inputs
through the data input latch, U10. After all five columns have been scanned, the row data is then
decoded to see which keys, if any, are pressed. The HHCH then waits for debounce until the keypad
is scanned again. The keypad is scanned every 25 ms. If the same key is still pressed, the key
register value is determined from a key table. If the SB9600 serial bus is not busy, a serial-bus
"BUTCTL" message transmission is initiated. The same process also applies when a key is
released.
3.1.4

Microphone Preamplifier and PTT

PTT is achieved by activating switch S1. This places a ground on the microphone pre-amplifier
circuit, comprising Q6 and associated components, and effectively turns it on. Activation of S7 also
places a low on U10-6 input, which is passed on to U3 and transmitted as an SB9600 message back
to the radio. When this message is received by the radio, (if required) an SB9600 message is sent
back to the microphone and detected by U3, and the PTT LED is activated through U8. However,
illumination of the PTT LED is under radio control and is therefore dependent upon radio
programming. Q7 is also connected across the PTT line. Its purpose is to remotely enable the
microphone pre-amplifier through a received SB9600 serial-bus message, as required.
September 26, 2003
Theory of Operation: W3 Control Head
6881096C77-O