Static Random Access Memory (Sram); Controller Board Audio And Signalling Circuits; Audio Signalling Filter Ic With Compander (Asfic Cmp) - Motorola CDM1550 LS+ Detailed Service Manual

2-8 Theory of Operation
U0101-69 is the high reference voltage for the AID ports on the IJP. Capacitor C0101 filters the +5
volt reference. If this voltage is lower than +5 volt, theA/Dreading is incorrect. Likewise U01 01, pin
68 is the low reference for the A/D ports. This line is normally tied to ground. If this line is not
connected to ground , the AID readings could be incorrect.
2.2.11 Static Random Access Memory (SRAM)
The SRAM (U0121) 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 U0122, pin 20 which is the result of U01 01-CSGP2 going low. U0122 is commonly referred to
as the external RAM as opposed to the internal RAM which is the 3 KBs of RAM (part of the
68HC11FLO). Both RAM spaces serve the purpose. However, the internal RAM is used for the
calculated values which are accessed most often.
Capacitor C0122 filters out any ac noise which may ride on +5V at U0122.
2.3 Controller Board Audio and Signalling Circuits
2.3.1 Audio Signalling Filter IC with Compander (ASFIC CMP)
The ASFIC CMP (U0221) used in the controller has the four following functions:
RX/TX audio shaping, i.e. filtering, amplification, attenuation
RX/TX signalling, PL/DPL/HST/MDC
Squelch detection
11P clock signal generation
The ASFIC CMP is programmable through the SPI BUS (U0221-20/21/22), normally receiving 19
bytes. This programming sets up various paths within the ASFIC CMP to route audio and/or
signalling signals through the appropriate filtering, gain, and attenuator blocks. The ASFIC CMP
also has six general control bits (GCB0-5) which are CMOS level outputs and used for NOISE
BLANKER (GCBO) in low band radios, EXTERNAL ALARM (GCB1), and DC POWER ON (GCB2)
to switch the voltage regulators (and the radio) on and off. GCB3 controls U0251, pin 11 to output
either RX FLAT AUDIO or RX FILTERED AUDIO on the accessory connector, pin 11. GCB4 controls
U0251, pin 10 to use either the external microphone input or the voice storage playback signal.
GCB5 switches the audio PA on and off.