2-14 THEORY OF OPERATION
5.9 Static Random Access Memory (SRAM)
The SRAM (U402) contains temporary radio calculations or parameters that can change very
frequently, and which are generated and stored by the software during its normal operation. The
information is lost when the radio is turned off.
The device allows an unlimited number of write cycles. SRAM accesses are indicated by the CS
signal U402 (which comes from U403-CSGP2) going low. U402 is commonly referred to as the
external RAM as opposed to the internal RAM which is the 3 Kilobytes of RAM which is part of the
68HC11FL0. Both RAM spaces serve the purpose. However, the internal RAM is used for the
calculated values which are accessed most often.
Capacitor C402 and C411 serves to filter out any AC noise which may ride on +3.3 V at U402
6.0 Control Board Audio and Signalling Circuits
6.1 Audio Signalling Filter IC and Compander (ASFIC CMP)
The ASFIC CMP (U504) used in the controller has the following four functions:
1. RX/TX audio shaping, i.e. filtering, amplification, attenuation
2. RX/TX signaling, PL/DPL/HST/MDC
3. Squelch detection
4. µP clock signal generation
The ASFIC CMP is programmable through the SPI BUS (U504 pins–20/21/22), normally receiving
19 bytes. This programming sets up various paths within the ASFIC CMP to route audio and/or
signaling signals through the appropriate filtering, gain and attenuator blocks. The ASFIC CMP also
has 6 General Control Bits GCB0–5 which are CMOS level outputs and used for the following:
• GCBO – BW Select
• GCBI – switches the audio PA On/Off
• GCB2 – DC Power On switches the voltage regulator (and the radio) on and off
• GCB3 – Control on MUX U509 pin 9 to select between Low Cost Mic path to STD Mic Path
• GCB4 – Control on MUX U509 pin 11 to select between Flat RX path to filtered RX path on
the accessory connector.
• GCB5 – Control on MUX U509 pin 10 to select between Flat TX path mute and Flat TX path
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