AN1154 STMicroelectronics, AN1154 Datasheet - Page 3

• Manufacturer Part Number: AN1154
• Description: 8031-PSD DESIGN TUTORIAL
• Manufacturer: STMicroelectronics

Figure 1. Block Diagram of Automatic Gain Control Circuit
The MCU could be used to perform this real-time gain adjustment, but this would leave it with little
execution time for other tasks. It is highly desirable to free up the MCU by off-loading these repetitive tasks
to dedicated hardware. The AGC function can be moved into the state machine, implemented using
programmable logic, as shown in Figure 1.
In this configuration, the MCU first loads the state machine with a desired signal level, and starts it running,
and then gets on with other tasks. Most of the time, the state machine works autonomously, reading the
outputs of the ADC and comparing the measured value with the desired value. The state machine only
needs to interrupt the MCU when the signal has drifted from the desired level. Along with the Interrupt line,
it provides two signals: ‘Trim’ and ‘Boost’. If the signal level from the receiver is too high, the interrupt is
accompanied by ‘Trim’, and the MCU writes the appropriate value to the PGA to decrement the gain.
Likewise, if the signal level is too low, the interrupt from the state machine is accompanied by ‘Boost’.
This tutorial shows how to implement this AGC function two different ways: a discrete IC solution (using
individual IC devices for programmable logic, memory, etc.), as shown in Figure 2, and an integrated PSD
solution, as shown in Figure 3. In addition to the AGC function, other features that have been implemented
include: the Real-Time-Clock (RTC), In-System Programmability (ISP), and miscellaneous I/O signals.
Please refer to Appendix F for information related to system memory mapping, ISP issues using a UART,
and memory paging considerations.
RF Signal In
Modulated
Amp
Pre-
LO
Filter
Amplifier
Control
Setting
Gain
80C31
MCU
PGA
4
RF Receiver
Env
Out
Desired Level
Interrupt
Boost
Trim
Loop AGC
Closed
Base Band Signal
Envelope Out
4
AN1154 - APPLICATION NOTE
Converter
Machine
State
A/D
4 of 8
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