Am7910 - FSK MODEM
GENERAL DESCRIPTIONThe Am79110 is a single-chip asynchronous Frequency Shift Keying (FSK) voicerband modem. It is pin selectable for baud rates of 300, 600 or 1200 bits per second and is compatible with the applicable Bell and CCITT recommended standards for 103/113/108, 202, V.21 and V.23 type modems. Five mode control lines select a desired modem configuration. Digital signal processsing techniques are employed in the Am7910 to perform all major funcitions such as modulation, demodulation and filtering. The Am7910 contains on-chip analog-to-digital and digital-to-analog converter circuits to minimize the external components in a system. This device includes the essential RS-232/CCITT V.24 terminal control signals with TTL levels. Clocking can be generated by attaching a crystal to drive the internal crystal oscillator or by applying an external clock signal. A data access arrangement (DAA) or acoustic coupler must provide the phone line interface externally. The Am7910 is fabricated using N-channell MOS technology in a 28-pin package. All the digital input and output signals (except the external clock signal) are TTL comptible. Power supply requirements are ±5 volts.
INTERFACE SIGNAL DESCRIPTION
MC0 MC4 (CONTROL INPUTS)These five inputs select one of thirlytwo modem configurations according to the Bell or CCITT specifications listed in Table 1. Only 19 of these 32 modes are actually available to the user.
Modes 0-8 are the normal operation modes The 1200 Baud modes can be selected with or without a compromise equalizer.
Modes 16-25 permit loop back of the Am7910 transmitter and receiver. No internal connection is made. The user must externally connect the TRANSMITTED CARRIER pin (figure 3) to the RECEIVED CARRIER pin if analog loopback is required. For digital loopback, external connection of RECEIVED DATA and TRANSMITTED DATA is required. Whenever a mode in this group is selected. the effect is to set all transmit and receive filters to the same channel frequency band so that loopback can be performed.
Modes 9-15 and 26-31 are reserved and should not be used.
DATA TERMINAL READY (DTR)A LOW level on this input indicates the data terminal desires to send and or receive data via the modem. This signal is gated with all other TTL inputs and outputs so that a low level enables all these signals as well as the internal control logic to function. A HIGH level disables all TTL I/O pins and the internal logic.
REOUEST TO SEND (RTS)A LOW level on this input instructs the modem to enter transmit mode. This input must remain LOW for the duration of data transmission. The signal has no effect if DATA TERMINAL READY is HIGH (disabled). A HIGH level on this input turns off the transmitter.
CLEAR TO SEND (CTS)This output goes LOW at the end of a delay initiated when REQUEST TO SEND goes LOW, Actual data to be transmitted should not be presented to the TRANSMITTED DATA inpul until a LOW is indicated on the CLEAR TO SEND output. Normally the user should force the TD input HIGH whenever CTS is off (HIGH). This signal never goes LOW as long as DTP is HIGH (disabled). CLEAR TO SEND goes HIGH at the end of a delay initiated when REQUEST TO SEND goes HIGH.
CARRIER DETECT (CD)A LOW on this output indicates that a valid carrier signal is present at the receiver and has been present for at least a time, tCDON where tCDON depends upon the selected modem configuration (Table 2). A HIGH on this output signifies that no valid carrier is being received and has not been received for a time, tCDOFF CARRIER DETECT remains HIGH when DTR is HIGH. Values for tCDON and tCDOFF are configuration dependent and are listed in Table 2.
TRANSMITTED DATA (TD)Data bits to be transmitted are presented on this input serially; HIGH (mark) corresponds to logic 1 and LOW (space) corresponds to logic 0. This data determines which frequency appears at any instant at the TRANSMITTED CARRIER output pin (Table 2). No.signal appears at the TRANSMITTED CARRIER output unless DTR is LOW and RTS is LOW.
RECEIVED DATA (RD)Data bits demodulated from the RECEIVED CARRIER input are available serially at this output. HIGH (mark) indicates logic 1 and LOW (space) indicates logic 0. Under the following condillions this output is lorced to logic I because the data may be invalid:
BACK REQUEST TO SEND (BRTS)Since the 1200 bps modem configurations, Bell 202 and CCITT V.23, permit only half duplex operation over two-wire lines, a low baud rate "backward" channel is provided for transmission from the main channel receiver to the main channel transmitter. This input signal (BRTS) is equivalent to REQUEST TO SEND for the main channel, except it belongs to the backward channel. Note that since the Am7910 contains a single transmitter, RTS and BRTS should not be asserted simultaneously. BRTS is meaningful only when a 202 or V.23 mode is selected by MC0-MC4. In all other modes it is ignored.
For V.23 mode the frequency appearing at the transmitted carrier (TC) output pin is determined by a MARK or SPACE at the back transmitted data (BTD) input (Table 2).
For 202 mode a frequency of 387Hz appears at TC when BRTS is LOW and BTD is HIGH. No energy (0.0 volts) appears at TC when BRTS is HIGH. BTD should be fixed HIGH for 202 back channel transmission. The signal, BRTS, then is equivalent to the signal. Secondary Request-to-Send, for 202 S/T modems, or Supervisory Transmitted Data for 202 C/D modems.
BACK CLEAR TO SEND (BCTS)This line is equivalent to CLEAR TO SEND for the main channel, except it belongs to the back channel, BCTS is meaningful only when a V.23 mode is selected by MC0-MC4. This signal is not used in Bell 202 back mode.
BACK CARRIER DETECT (BCD)This line is equivalent to CARRIER DETECT for the main channel, except it belongs to the backward channel. BCD is meaningful only when a 202 or V.23 mode is selected by MC0-MC4. For V.23 back channel mode, BCD turns on when either the MARK or SPACE frequency appears with sufficient level at the received carrier (RC) input.
For 202 back channel mode, BCD turns on in response to a 387Hz tone of sufficient level at the RC input. In this case BCD is equivalent to the signal. Secondary Received Line Signal Detector, for 202 S/T modems, or Supervisory Received Data for 202 C/D modems.
BACK TRANSMITTED DATA (BTD)This line is equivalent to TRANSMITTED DATA for the main channel, except it belongs to the back channel. BTD is meaningful only when a 202 or V.23 mode is selected by MC0-MC4. For 202 back transmission of on/off keying, BTD should be fixed at a HIGH level.
BACK RECEIVED DATA (BRD)This line is equivalent to RECEIVED DATA (except clamping) for the main channel, except it belongs to the back channel. BRD is meaningful only when a V.23 mode is selected by MC0-MC4. Under the following conditions this output is forced HIGH:
TRANSMITTED CARRIER (TC)This analog output is the modulated carrier to be conditioned and sent over the phone line.
RECEIVED CARRIER (RC)This input is the analog signal received from the phone line. The modem extracts the information contained in this modulated carrier and converts it into a serial data stream for presentation at the RECEIVED DATA (BACK RECEIVED DATA) output.
RINGThis input signal permits auto-answer capability by responding to a ringing signal from a data access arrangement. It a ringing signal is detected (RING LOW) and DTR is LOW, the modem begins a sequence to generate an answer tone at the TC output.
XTAL1, XTAL2Master timing of the modem is provided by either a crystal connected to these two inputs or an external clock inserted into XTAL1. The value of the crystal or the external clock frequency must be 2.4576MHz ±.01%.
VCC+ 5 volt power supply. (±5%)
VBB- 5 volt power supply. (±5%)
DGNDDigital signal ground pin
AGNDAnalog signal ground pin (for TRANSMITTED CARRIER and RECEIVED CARRIER)
CAP1, CAP2Connection points of external capacitor/resistor required for proper operation of on-chip analog-to-digital converter.
RESETThis input signal is for a reset circuit which operates in either of two modes. It automatically resets when power is applied to the device, or it can be activated by application of an external active low TTL pulse.
THEORY OF OPERATIONThe Am7910 MODEM consists of three main sections, shown in the block diagram of Figure 2 - Transmitter, Receiver, and Interface Control.
TRANSMITTER (Modulator)The transmitter, shown in Figure 4, receives binary digital data from a source such as a UART and converts the data to an analog signal using frequency shift keying (FSK) modulation. This analog signal is applied to the phone line through a DAA or acoustic coupler. FSK is a modulation technique which encodes one bit per baud. A logic one applied to the TRANSMITTED DATA (TD) input causes a sine wave at a given frequency to appear at the analog TRANSMITTED CARRIER (TC) output. A logic zero applied to input TD causes a sine wave of a different frequency to appear at the TC output. As the data at the TD input switches between logical one and zero, the TC output switches between the two frequencies. In the Am7910 this switching between frequencies is phase continuous. The frequencies themselves are digitally synthesized sine functions.
The frequencies for each modem configuration available in the Am7910 are listed in Table 3a.
The process of switching between two frequencies as in FSK
generates energy at many more frequencies than the two used in
the modulation. All the transmitted information can be recovered
from a frequency band 8 Hz wide, where 8 is the bit rate or
maximum rate of change of the digital data at the TD input This
band is centered about a frequency, fC,
In addition to this primary information band there exist side bands containing redundant information. It is desirable to attenuate these bands for two reasons:
RECEIVER (Demodulator)A simplified block diagram of the Am7910 FSK receiver is shown in Figure 5. Data transmitted from a remote site modem over the phone line is an FSK-modulated analog carrier. This carrier is applied to the RECEIVED CARRIER (RC) pin via a DAA or acoustic coupler. The first stage of the demodulator is a simple on-chip analog low pass anti-alias filter. The output of this is converted into digital form and filtered by digital bandpass filters to improve the signal to noise ratio and reject other independent channel frequencies associated with the phone line in the case of full duplex configuration. The bandpass filtered output is digitally demodulated to recover the binary data. A carrier detect signal is also digitally extracted from the received line carrier to indicate valid data.
Am7910 - FSK MODEM