| ISA Interface The PCV100 decodes A9..A4 for
the base address using a LS688 IC. A LS139 decodes A3..A2 to generate
internal SEL2..SEL0 chip select signals. A1..A0 are connected to
an 8255 Peripheral Adapter, an 8253 timer, and another LS139 to
decode the CL3..CL0 (clear interrupt) signals. The base address
is user selectable via a DIP switch.
SEL2..SEL0 Definitions
A3 A2
0 0 SEL0 - 8255
0 1 SEL1 - 8253
1 0 SEL2 - CL3..CL0
1 1 unused
8255 Port Definitions
The 8255 is configured with port A as output, port B as input and
port C as split input/output. The port pin definitions follow:
PA0 - OFFHOOK - Telephony module hook control
PA1 - TONEOFF - DTMF transceiver tone generator control
PA2 - ENARING - Enable telephony ring detect interrupt
PA3 - ENATONE - Enable DTMF transceiver tone detect interrupt
PA4 - ENAI3 - Enable spare interrupt
PA5 - INPUTSEL - Select telephone or mic as record input
PA6 - unused
PA7 - unused
PB0 - unused
PB1 - unused
PB2 - unused
PB3 - CALLPRG - DTMF call progress tone detect
PB4 - /INT3 - Spare interrupt status
PB5 - /INT2 - Tone detect interrupt status
PB6 - /INT1 - Ring detect interrupt status
PB7 - /INT0 - Swap (DMA terminal count) interrupt
PC3..PC0 4-bit DTMF transceiver tone detect code
PC7..PC4 4-bit DTMF transceiver tone generate code
8253 Timer
The 8253 has three counters that can be configured in a variety
of ways. For the PCV100, the first two counters are set to divide
the input clock to a lower frequency. The third counter is unused.
The input to counter 1 is a 2MHz clock derived from the 8MHz ISA
bus clock. The output of counter 1 is feed to the input of counter
2. The output of counter 2 is an 8kHz clock to drive the 8-bit ADC
and DAC chips. The output of counter 1 drives two switched capacitor
filters that provide anti-alias and anti-image filtering.
DMA
The PCV100 is uses the 8-bit system DMA channel 0 for record and
playback operation. At each transition of the 8kHz sample clock,
high is latched onto the DREQ0 line. The /DACK0 line is OR’ed
with the /IOW line to latch data to the DAC and OR’ed with
the /IOR line to get data from the ADC. The /DACK0 line also clears
the DREQ0 latch. TC and /DACK0 are OR’ed to produce the SWAP
interrupt.
Interrupts
The PCV100 uses IRQ10. Three interrupts can be generated by the
hardware: Ring Detect, Tone Detect, and SWAP. These interrupts are
AND’ed together as input to the IRQ10 latch. The ISR software
must handle the interrupt by reading port B of the 8255 to determine
the PCV100 interrupt source and then acknowledging the interrupt
by reading the appropriate CLx line.
Telephony interface
A XECOM XE0022V FCC approved telephony interface module is used
to interface to the PSTN. This module has logic level signals for
ringing signal detect and hook control.
DTMF transceiver
A 75T2080 DTMF transceiver chip generates DTMF tones for dialing
a number and also detects and decodes received DTMF tones. In additions,
there is a call progress (dial tone, busy signal, reorder tone etc.)
detect line. This IC requires a standard 3.58MHz color TV crystal
for operation.
Analog switch
Two 4066 CMOS analog switches are used. One is used to select the
output of the DAC or the output of the DTMF tone generator for the
transmit input of the telephony interface. The other switch selects
either the line level input or the telephony receive output as the
input to the ADC for recording.
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