Chapter 4 Theory of Operation
Floating Logic
88
Floating Logic
Referring to the schematic shown on page 132, the floating common logic
controls operation of the entire instrument. All output functions and bus
command interpretation is performed in the main controller U19. The front
panel and the earth referenced logic operate as slaves to U19. The floating
common logic is comprised of the main controller U19, custom gate array U20,
the program ROM U13, RAM U14, calibration EEPROM U15, and the 12 MHz
clock oscillator. Non-volatile EEPROM U15 stores calibration constants,
calibration secure code, calibration count, and store/recall variables. Power-
on reset is provided to the main controller by the voltage regulator U10.
The main controller U19 is a 16-bit micro controller. It controls such features
as receive and transmit serial port, timer/counter ports, and selectable input
10-bit successive approximation A-to-D converter ports.
A conventional address/data bus is used to transfer data between the main
controller and external ROM and RAM. When the address latch enable (ALE)
signal goes high, address data is present on the address/data bus. ASIC U20
latches the address data and decodes the correct chip enable (low true) for
external ROM and RAM accesses and for read/write accesses to the internal
registers of U20. The system memory map is shown below.
Program ROM U13 contains four 64k x 8 data banks of data. Banks are selected
by controlling A16 and A17 ROM address bits directly from the main controller
port bits.
Custom gate array U20 performs address latching and memory map decoding
functions as discussed above. In addition, U20 contains a variety of internal
read/write registers. The read (XRD) and write (XWR) signals transfer data out
of and into U20 when it is addressed. There are four internal registers in U20:
an internal configuration register, an 8 bit counter register, a serial transmit/
receive register, and an internal status register.
The counte r r e g ister is used to captur e th e A D C slop e c o u nt at the COMP input.
The COMP input functions as both a clocked comparator and the slope counter
input for the ADC. In both cases the counter register captures the lower 8 bits
of a 24-bit counter. The upper 16 bits of the count are captured by the SYNC
input to U19.
0000
H
- 1FF7
H
U14 32k x 8 RAM
1FF8
H
- 1FFF
H
U20 Gate Array
2000
H
- FFFF
H
U13 Program ROM
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