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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-21
Microprocessor Interface. The Microprocessor Interface is used by the Micropro-
cessor board to communicate with the Reference Oscillator board. An 8 bit input
shift register controls the operation of the board, and an 8 bit output shift register
contains frequency correction bits for indicating the Oscillator frequency. The
microprocessor clocks (CLKO) eight bits of serial data (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 (see Figure 3--8 on page 3--17) and
using CLKI-- to clock serial DATAI out. Refer to Table 3--1 for the bit mapping
of the registers.
Bit 0 of the input register is the last bit to leave the microprocessor, and bit 0 of
the output register is the last bit to reach the microprocessor.
Bit 0 of the input register enables the calibrator signal when high and disables it
when low. Bit 7 of the input register allows the microprocessor to read the
contents of the output register when set low, and it tristates the output buffer
when set high.
Table 3- 1: Input and Output Bit Definition
Input Register Output Register
Bit Function Bit Function
0 Cal Enable 0 CB0 1
1 None 1 CB1 2
2 None 2 CB2 4
3 None 3 CB3 8
4 None 4 CB4 16
5 None 5 CB5 32
6 None 6 CB6 64
7 Output Enable 7 CB7 128
The output register has 8 bits available to represent the Oscillator frequency. The
microprocessor can then read the Oscillator frequency through these bits to
within 10 Hz. The possible values are 1 through 254. The microprocessor
interprets a value of 128 as a frequency of 100 MHz, 127 as 99,999,990 Hz, 129
as 100,000,010 Hz, and so forth.
Values 0 (all bits low) and 255 (all bits high) are used to send a hardware failure
message to the microprocessor.
Phase Lock Assembly (Center Frequency Control System)
There are three major operating areas of the frequency control system: