Theory of Operation
2715 Spectrum Analyzer Service Manual 3-21
Microprocessor Interface. The Microprocessor Interfaceis used by the Micropro-
cessor board to communicatewith the Reference Oscillator board. An 8 bit input
shift register controls theoperation of the board, and an 8 bit output shift register
contains frequency correctionbits for indicating the Oscillator frequency. The
microprocessor clocks (CLKO) eight bits of serialdata (DATAO) into the input
serial register,and it then latches the serial contents (RFLATCH--) to the parallel
outputs. The microprocessor clocks data out of the output register by setting bit 7
of the input register low to enable the Buffer (seeFigure 3 --8 on page 3--17) and
using CLKI-- to clock serial DATAI out. Refer to Table 3--1for the bit mapping
of the registers.
Bit 0 of the input register is the last bit to leavethe microprocessor, and bit 0 of
the output register is the l ast bit to reach the microprocessor.
Bit 0 of the input register enablesthe calibrator signal when high and disables it
when low. Bit 7 of the input registerallows the microprocessor to read the
contents of the output registerwhen set low, and it tristates the output buffer
when set high.
Table3 -1: Input and Output Bit Definition
Input Register Output Register
Bit Function Bit Function
0Cal Enable 0CB0 1
1None 1CB1 2
2None 2CB2 4
3None 3CB3 8
4None 4CB4 16
5None 5CB5 32
6None 6CB6 64
7Output Enable 7CB7 128
The output register has 8 bits available to represent the Oscillatorfrequency. The
microprocessor canthen read the Oscillator frequency through these bits to
within 10 Hz. The possible values are 1 through254. The microprocessor
interprets a value of128 as a frequency of 100 MHz, 127 as 99,999,990 Hz, 129
as 100,000,010 Hz, and so forth.
Values0 (all bits low) and 255 (all bits high) are used to send a hardware failure
message to the microprocessor.
Phase Lock Assembly (Center Frequency ControlSystem)
There are three majoroperating areas of the frequency control system: