Keithley 486 Instruction Manual page 154

SECTTON 6
Theoy of
operation
6.5.2 Microcomputer
The microcomputer controls all operating aspects of the
instrument. The various sstions of the miaocomputer
include the 68Bo9 MPU, the memory circuits, and I/O
(GI'L4) circuits.
Microprocessor
U.509 is a 68809 8-bit microprocessor that controls the in-
strument. The processor has an Bbit data bus (D7-DO), a
X-bit address bus (A15-AO), as well as a number of con-
trol lines.
Y500 is an 8MHz crystal that provides a time base for the
inte~MpUdodcTheclodcsignalisinternally divided
downto2MHzandisusedasabusdodcviatheEandQ
lines.
Power-on reset for the MPU is provided by U513. The re-
set signal is also used to inhibit writig to the EEROM
during power-up.
Memory Circuits
U508, a &JK byte PROM which uses 48k bytes, contains
the instmction ding for the MPLJ. working storage for
the MPLJ is provided by U511, an SK byte RAM. Non-
volatile storage for data such as calibration constants,
zero-corredioi constants and power-on defaults is pro-
vided by EEPROM U510.
Address Decoder
U507 is a PAL that performs the address decoding for the
various memory and I/O &xi&., and is also instruman-
tal in the operation of the display serial pofi u507 moni-
tors the states of the AlsAl address lines, and it selects
the appropriate section of the memory map accordingly.
A memory map is shown in Figure 6-14.
I/o Port
U504, a 6522 VL4 (Versatile Interface Adapter) provides
much of the I/O for the micmcomputer. Port A reads
keyboard and knob data, while port B provides a serial
port for controlling the analog board, and a counter for
counting A/D counts from the analog board.
1 FFF
2000
EEROM
us10
i
27FF
VIA U504
EFF> 16 Register Addresses
GPIA U514 ;;;; > 8 Register Addresses
Display US06 iFF!F) One Byte for Display
Slatus U502 4ooo
or status
48KB
ROM
FFFF
Figure 6-14. Memory tip
Display Serial Port
The display serial port provides high-speed serial data
transmission to the display and keyboard circuits located
on the display board. Figure 6-15 shows a simplified
block diagram of the serial port.
The Zh4H.z clock (GCLK) for the port is provided by the
PAL, U507. U506 is a parallel-&serial converter used to
shift out display and keyboard data to circuits located on
the display board. U503 is a bit-shift counter and pm
v-ides the SHIFI DONE signal when all eight bits of U506
are shifted out at the ZMHz clock rate.
The basic data transmission sequence is as follows:
1. The byte
to
be tmnsmitted is written to U506. The act
of writing this byte only parallel loads U506, but it
also clears the shit? counter, U503, via its CLR line.
clearing U503 enables the GCLK (gated clock) signal
supplied by U507. GCLK becomes the clock signal
after inversion by U502.
2. The GCLK pulses applied to U506 cause the 8-bit
data word to be shifted out the DISPLAY DATA line.
At the same time, the display clock pulses cause the
shift counter, u503, to count up, one count per clock
cycle.
6-11