TCM29C23, TCM129C23 Variable-Frequency PCM or DSP Interface: Description
TCM29C23, TCM129C23 Variable-Frequency PCM or DSP Interface: Description
TCM29C23, TCM129C23 Variable-Frequency PCM or DSP Interface: Description
D D D D D D D D
Combined ADC, DAC, and Filters Extended Variable Frequency Operation Master Clock Up to 4.096 MHz Sample Rates Up to 16 kHz Passband Up to 6 kHz Reliable Silicon-Gate CMOS Technology Low Power Consumption Operating Mode . . . 80 mW Typical Power-Down Mode . . . 5 mW Typical Excellent Power-Supply Rejection Ratio Over Frequency Range of 0 to 50 kHz No External Components Needed for Sample, Hold, and Autozero Functions Precision Internal Voltage References -law and A-law Coding
VBB PWRO + PWRO GSR PDN CLKSEL DCLKR PCM IN FSR/TSRE DGTL GND
1 2 3 4 5 6 7 8 9 10
20 19 18 17 16 15 14 13 12 11
VCC GSX ANLG IN ANLG IN + ANLG GND SIGX/ASEL TSX/DCLKX PCM OUT FSX/TSXE CLKR/CLKX
description
The TCM29C23 and TCM129C23 are single-chip PCM codecs (pulse-code-modulated encoders and decoders) and PCM lines filters. These devices provide all the functions required to interface a full-duplex (4-wire) voice telephone circuit with a TDM (time-division-multiplexed) system. Primary applications include digital encryption systems, digital voice-band data storage systems, digital signal processing, and mobile telephones. These devices are designed to perform the transmit encoding (A/D conversion) and receive decoding (D/A conversion) as well as the transmit and receive filtering functions in a pulse-code-modulated system. They are intended to be used at the analog termination of a PCM line or trunk. The TCM29C23 and TCM129C23 provide the band-pass filtering of the analog signals prior to encoding and after decoding. These combination devices perform the encoding and decoding of voice and call progress tones as well as the signaling supervision information. The TCM29C23 is characterized for operation from 0C to 70C. The TCM129C23 is characterized for operation from 40C to 85C.
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates.
PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters.
13 ANLG IN + ANLG IN GSX 17 18 19 Analogto-Digital Control Logic Control Section Filter GSR 4 Gain Set Control Logic Buffer PWRO+ 2 Sample and Hold and DAC Reference 20 VCC 1 VBB 10 DGTL GND 16 ANLG GND 9 FSR/TSRE Digitalto-Analog Control Logic 8 Input Register 7 6 5 12 11 Filter Sample and Hold and DAC Comparator Successive Approximation Output Register 14
15
SIGX/ASEL
Reference
FSX/TSXE CLKR/CLKX
Receive Section
CLKSEL PDN
PCM IN DCLKR
PWRO
Terminal Functions
TERMINAL NAME ANLG GND ANLG IN + ANLG IN CLKR CLKSEL CLKX DCLKR NO. 16 17 18 11 6 11 7 I I I I I I I/O DESCRIPTION Analog ground return for all internal voice circuits. Not internally connected to DGTL GND. Noninverting analog input to uncommitted transmit operational amplifier. Inverting analog input to uncommitted transmit operational amplifier. Receive master clock and data clock for the fixed-data-rate mode. Receive master clock only for variable-data-rate mode. CLKR and CLKX are internally connected together. Clock frequency selection. Input must be connected to VBB, VCC, or ground to reflect the master clock frequency. Transmit master clock and data clock for the fixed-data-rate mode. Transmit master clock only for variable-data-rate mode. CLKR and CLKX are internally connected. Selects fixed- or variable-data-rate operation. When connected to VBB, the device operates in the fixed-data-rate mode. When DCLKR is not connected to VBB, the device operates in the variable-data-rate mode and DCLKR becomes the receive data clock, which operates at frequencies from 64 kHz to 4.096 MHz. Digital ground for all internal logic circuits. Not internally connected to ANLG GND. I Frame-synchronization clock input/time-slot enable for receive channel. In the fixed-data-rate mode, FSR distinguishes between signaling and nonsignaling frames by a double- or single-length pulse, respectively. In the variable-data-rate mode, this signal must remain high for the duration of the slot. The receive channel enters the standby state when FSR is TTL low for 300 ms. Frame-synchronization clock input/time-slot enable for the transmit channel. Operates independently of, but in an analogous manner to, FSR/TSRE. The transmit channel enters the standby state when FSX is low for 300 ms. Input to the gain-setting network on the output power amplifier. Transmission level can be adjusted over a 12-dB range depending upon the voltage at GSR. Output terminal of internal uncommitted operational amplifier. Internally, this is the voice signal input to the transmit filter. Receive PCM input. PCM data is clocked in on this pin on eight consecutive negative transition of the receive data clock, which is CLKR in fixed-data-rate timing and DCLKR in variable-data-rate timing. Transmit PCM output. PCM data is clocked out of this output on eight consecutive positive transition of the transmit data clock, which is CLKX in fixed-data-rate timing and DCLKX in variable-data-rate timing. Power-down select. This device is inactive with a TTL low-level input to this terminal and active with a TTL high-level input to this terminal. Noninverting output of power amplifier. Can drive transformer hybrids or high-impedance loads directly in either a differential or single-ended configuration. Inverting output of power amplifier; functionally identical to but complementary to PWRO +. A-law and -law operation select. When connected to VBB, A-law is selected. When connected to VCC or ground, -law is selected. Transmit channel time slot strobe (output) or data clock (input) for the transmit channel. In the fixed-data-rate mode, this is an open-drain output to be used as an enable signal for a 3-state output buffer. In the variable-data-rate mode, DCLKX becomes the transmit data clock, which operates at TTL levels from 64 kHz to 2.048 MHz. Most negative supply voltage; input is 5 V 5%. Most positive supply voltage; input is 5 V 5%.
10 9
FSX/TSXE GSR GSX PCM IN PCM OUT PDN PWRO + PWRO SIGX/ASEL TSX/DCLKX
12 4 19 8 13 5 2 3 15 12
I I O I O I O O I I/O
VBB VCC
1 16
absolute maximum ratings over operating free-air temperature range (unless otherwise noted)
Supply voltage range, VCC (see Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to 15 V Output voltage range, VO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to 15 V Input voltage range, VI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to 15 V Digital ground voltage range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to 15 V Operating free-air temperature range, TA: TCM29C23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to70C TCM129C23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40C to 85C Storage temperature range, Tstg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65C to 150C Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds: DW or N package . . . . . . . . . . . . . . . 260C
Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. NOTE 1: Voltage values for maximum ratings are with respect to VBB.
NOTES: 2. To avoid possible damage to these CMOS devices and resulting reliability problems, the power-up procedure described in the device power-up sequence paragraphs later in this document should be followed. 3. Voltages at analog inputs and outputs, VCC and VBB, are with respect to ANLG GND. All other voltages are referenced to DGTL GND unless otherwise noted.
electrical characteristics over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted)
supply current, fDCLK = 4.096 MHz, outputs not loaded
PARAMETER Operating ICC Supply current f S l t from VCC Standby Power down Operating IBB Supply current f S l t from VBB Standby Power down Operating PD Power dissipation Standby Power down FSX or FSR at VIL after 300 ms PDN at VIL after 10 s All typical values are at VBB = 5 V, VCC = 5 V, and TA = 25C. FSX or FSR at VIL after 300 ms PDN at VIL after 10 s FSX, or FSR at VIL after 300 ms PDN at VIL after 10 s TEST CONDITIONS TCM29C23 MIN TYP MAX 7 0.5 0.3 7 0.5 0.3 70 5 3 9 1 0.8 9 1 0.8 90 10 8 TCM129C23 MIN TYP MAX 8 0.7 0.4 8 0.7 0.4 80 7 4 13 1.5 1 13 1.5 1 130 15 10 mW mA mA UNIT
digital interface
PARAMETER VOH VOL IIH IIL Ci High-level output voltage PCM OUT TEST CONDITIONS IOH = 9.6 mA IOL = 3.2 mA VI = 2.2 V to VCC VI = 0 to 0.8 V 5 5 MIN TMC29C23 TYP MAX MIN TMC129C23 TYP MAX UNIT V 0.5 12 12 5 5 10 V A A pF pF
2.4 0.4 10 10 10
2.4
Low-level output voltage at PCM OUT, TSX, SIG High-level input current, any digital input Low-level input current, any digital input Input capacitance
Co Output capacitance All typical values are at VBB = 5 V, VCC = 5 V, and TA = 25C.
gain and dynamic range, VCC = 5 V, VBB = 5 V, TA = 25C (unless otherwise noted) (see Notes 4, 5, and 6)
PARAMETER Encoder milliwatt response (transmit gain tolerance) Encoder milliwatt response (nominal supplies and temperature) Digital milliwatt response (receive tolerance gain) relative to zero-transmission level point Digital milliwatt response variation with temperature and supplies -law Zero transmission level point transmit channel (0 dBm0) Zero-transmission-level point, A-law -law A-law -law Zero-transmission-level point, Zero transmission level point receive channel (0 dBm0) A-law -law A-law TEST CONDITIONS Signal input = 1.064 Vrms for -law Signal input = 1.068 Vrms for A-law TA = 0C to 70C, Supply voltages = 5 V 5% Signal input per CCITT G.711, Output signal = 1 kHz TA = 0C to 70C, Supply voltages = 5 V 5% RL = 600 RL = 900 RL = 600 RL = 900 2.76 2.79 1 1.03 5.76 5.79 4 4.03 dBm dBm 0.5 MIN TYP 0 5 0.5 MAX 1 0.15 1 0.15 UNIT dBm0 dB dBm0 dB
NOTES: 4. Unless otherwise noted, the analog input is a 0-dBm0, 1020-Hz sine wave, where 0 dBm0 is defined as the zero-reference point of the channel under test. This corresponds to an analog signal input of 1.064 Vrms or an output of 1.503 Vrms. 5. The input amplifier is set for noninverting unity gain. The digital input is a PCM bit stream generated by passing a 0-dBm0, 1020-Hz sine wave through an ideal encoder. 6. Receive output is measured single ended in the maximum gain configuration. To set the output amplifier for maximum gain, GSR is connected to PWRO and the output is taken at PWRO+. All output levels are (sin x)/x corrected.
gain tracking over recommended ranges of supply voltage and operating free-air temperature, reference level = 10 dBm0
PARAMETER Transmit gain tracking error sinusoidal input gain-tracking error, Receive gain tracking error sinusoidal input gain-tracking error, TEST CONDITIONS 3 input level 40 dBm0 40 > input level 50 dBm0 3 input level 40 dBm0 40 > input level 50 dBm0 MIN MAX 0.5 1.5 0.5 1.5 UNIT dB dB
noise over recommended ranges of supply voltage and operating free-air temperature range
PARAMETER Transmit noise, C-message weighted Transmit noise, psophometrically weighted Receive noise, C-message-weighted quiet code Receive noise, psophometrically weighted TEST CONDITIONS ANLG IN+ = ANLG GND, ANLG IN+ = ANLG GND, PCM IN = 11111111 (-law), Measured at PWRO + ANLG IN = GSX ANLG IN = GSX PCM IN = 10101010 (A-law), MIN MAX 18 72 11 79 UNIT dBrnC0 dBm0p dBrnC0 dBm0p
power-supply rejection and crosstalk attenuation over recommended ranges of supply voltage and operating free-air temperature
PARAMETER VCC supply-voltage rejection ratio, y g j , transmit channel VBB supply-voltage rejection ratio, y g j , transmit channel y g j , VCC supply-voltage rejection ratio, receive channel (single ended) VBB supply-voltage rejection ratio, receive y g j , channel (single ended) 0 f < 30 kHz 30 f < 50 kHz 0 f < 30 kHz 30 f < 50 kHz 0 f < 30 kHz 30 f < 50 kHz 0 f < 30 kHz 30 f < 50 kHz TEST CONDITIONS Idle channel, Supply signal = 200 mV peak to peak, peak f measured at PCM OUT Idle channel, Supply signal = 200 mV peak to peak, peak f measured at PCM OUT Idle channel, Supply signal = 200 mV peak to peak, peak f measured at PWRO + Idle channel, Supply signal = 200 mV peak to peak, peak Narrow-band, f measured at PWRO + ANLG IN+ = 0 dBm0, f = 1.02 kHz, Unity gain, PCM IN = lowest decode level, Measured at PWRO + PCM IN = 0 dBm0, Measured at PCM OUT f = 1.02 kHz, MIN TYP 30 dB 45 30 dB 55 20 dB 45 20 dB 45 MAX UNIT
68
dB
68
dB
distortion over recommended ranges of supply voltage and operating free-air temperature
PARAMETER Transmit signal-to-distortion ratio, sinusoidal i T it i l t di t ti ti i id l input t (CCITT G.712 Method 2) G 712 TEST CONDITIONS 0 ANLG IN+ 30 dBm0 30 > ANLG IN+ 40 dBm0 40 > ANLG IN+ 45 dBm0 0 ANLG IN+ 30 dBm0 Receive signal-to-distortion ratio, sinusoidal i R i i l t di t ti ti i id l input t (CCITT G.712 Method 2) G 712 Transmit single-frequency distortion products 30 > ANLG IN+ 40 dBm0 40 > ANLG IN+ 45 dBm0 AT&T Advisory #64 (3.8), Input signal = 0 dBm0 Input signal = 0 dBm0 MIN 33 28 23 33 28 23 40 46 dBm0 dBm0 dB dB TYP MAX UNIT
Receive single-frequency distortion products AT&T Advisory #64 (3.8), All typical values are at VBB = 5 V, VCC = 5 V, and TA = 25C.
transmit filter transfer over recommended ranges of supply voltage and operating free-air temperature, fDCLK = 4.096 MHz, FSX/FSR = 16 kHz (see Figure 1)
PARAMETER TEST CONDITIONS 50 Hz 200 Hz Gain relative to gain at 1.02 kHz In ut amplifier Input am lifier set for unity gain, Noninverting maximum gain output, Input signal at ANLG IN + is 0 dBm0 300 Hz to 6 kHz 6.5 kHz 6.8 kHz 8 kHz 9 kHz and above MIN 10 1 0.5 4 6 MAX 0 0.5 0.5 0.3 0 12 30 dB UNIT
receive filter transfer over recommended ranges of supply voltage and operating free-air temperature (see Figure 2)
PARAMETER TEST CONDITIONS Below 200 Hz 200 Hz 300 Hz to 6 kHz Gain relative to gain at 1.02 kHz Input signal at PCM IN is 0 dBm0 6.6 kHz 6.8 kHz 8 kHz 9.2 kHz and above MIN 2 1 0.5 4 6 MAX 0.5 0.5 0.5 0.3 0 12 30 dB UNIT
timing requirements
clock timing requirements over recommended ranges of supply voltage and operating free-air temperature (see Figure 3)
MIN tc(CLK) tr, tf tw(CLK) tw(DCLK) Clock period, for CLKX, CLKR (2.048-MHz systems) Rise and fall times for CLKX and CLKR Pulse duration for CLKX and CLKR (see Note 7) Pulse duration, DCLK (fDCLK = 64 kHz to 2.048 MHz) (see Note 7) Clock duty cycle, [tw(CLK)/tc(CLK)] for CLKX and CLKR All typical values are at VBB = 5 V, VCC = 5 V, and TA = 25C. NOTE 7: FSX CLK must be phase locked with CLKX. FSR CLK must be phase locked with CLKR. 244 5 110 110 45% 50% 55% 20 TYP MAX UNIT ns ns ns ns
transmit timing requirements over recommended ranges of supply voltage and operating free-air temperature, fixed-data-rate mode (see Figure 3)
PARAMETER td(FSX) Frame-sync delay time MIN 60 MAX tc(CLK) 60 UNIT ns
receive timing requirements over recommended ranges of supply voltages and operating free-air temperature, fixed-data-rate mode (see Figure 4)
PARAMETER td(FSR) Frame-sync delay time MIN 60 MAX tc(CLK) 60 UNIT ns
transmit timing requirements over recommended ranges of supply voltage and operating free-air temperature, variable-data-rate mode (see Figure 5)
PARAMETER td(TSDX) td(FSX) tc(DCLKX) Time-slot delay time from DCLKX Frame-sync delay time Clock period for DCLKX MIN 60 60 244 MAX td(DCLKX) 60 tc(CLK) 60 15620 UNIT ns ns ns
receive timing requirements over recommended ranges of supply voltages and operating free-air temperature, variable-data-rate mode (see Figure 6)
PARAMETER td(TSDR) td(FSR) tsu(PCM IN) th(PCM IN) tc(DCLKR) tSER Time-slot delay time from DCLKR Frame-sync delay time Setup time before bit 7 falling edge Hold time after bit 8 falling edge Data clock frequency Time-slot end receive time MIN 60 60 10 60 244 0 15620 MAX td(DCLKR)140 tc(CLK) 60 UNIT ns ns ns ns ns ns
switching characteristics
propagation delay times over recommended ranges of operating conditions, fixed-data-rate mode (see Figure 3)
PARAMETER tpd1 tpd2 tpd3 tpd4 tpd5 From rising edge of transmit clock to bit 1 data valid at PCM OUT (data enable time on time-slot entry) (see Note 8) From rising edge of transmit clock bit n to bit data valid at PCM OUT (data valid time) From falling edge of transmit clock bit 8 to bit 8 Hi-Z at PCM OUT (data float time on time-slot exit) (see Note 8) From rising edge of transmit clock bit 1 to TSX active (low) (time slot enable time) From falling edge of transmit clock bit 8 to TSX inactive (high) (time-slot disable time) (see Note 8) TEST CONDITIONS CL = 0 to 100 pF CL = 0 to 100 pF CL = 0 CL = 0 to 100 pF CL = 0 MIN 0 0 60 0 60 MAX 90 90 215 90 190 UNIT ns ns ns ns ns
NOTE 8: Timing parameters tpd1, tpd3, and tpd5 are referenced to the high-impedance state.
propagation delay times over recommended ranges of operating conditions, variable-data-rate mode (see Note 9 and Figure 5)
PARAMETER tpd7 tpd8 tpd9 tpd10 Data delay time from DCLKX Data delay from time-slot enable to PCM OUT Data delay from time-slot disable to PCM OUT Data delay time from FSX CL = 0 to 100 pF TEST CONDITIONS MIN 0 0 0 0 MAX 90 50 80 90 UNIT ns ns ns ns
td(TSDX) = 80 ns NOTE 9: Timing parameters tpd8 and tpd9 are referenced to the high-impedance state.
PARAMETER MEASUREMENT INFORMATION CLK, CLKR, and CLKX selection requirements for DSP-based applications
CLK, CLKR, and CLKX must be selected as follows:
CLKSEL PIN 5 V 0V 5V CLK, CLKR, CLKX (BETWEEN 1 MHz to 3 MHz) = (256) (frame-sync frequency) = (193) (frame-sync frequency) = (192) (frame-sync frequency)
10
0 dB 50 Hz 0
10 10 dB 50 Hz 20
10
20
30
30 30 dB 9000 Hz 40
40
50
50
60 10
50
100 f Frequency Hz
1k
60 10 k
Expanded Scale 11
0 dB 50 Hz
0 dB 6800 Hz 5 0.5 dB 200 Hz 0 1 dB 200 Hz 0.5 dB 300 Hz 0.5 dB 6000 Hz 4 dB 6600 Hz 6 dB 6800 Hz 5 0.5 dB 300 Hz Expanded Scale 0.3 dB 6800 Hz 0.5 dB 6000 Hz 0 5
10
10
20
20
30
30
40
40
50 10 k
12
2 tpd2
Bit 2
7 tpd3
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Bit 8
tpd5
Bit 1 = MSB = sign bit and locked in first on PCM IN or clocked out first on PCM OUT. Bit 8 = LSB = least significant bit and is clocked in last on PCM IN or is clocked out last on PCM OUT. NOTE A: Inputs are driven from 0.45 V to 2.4 V. Time intervals are referenced to 2 V if the high level is indicated and 0.8 V if the low level is indicated.
Bit 1 = MSB = sign bit and locked in first on PCM IN or clocked out first on PCM OUT. Bit 8 = LSB = least significant bit and is clocked in last on PCM IN or is clocked out last on PCM OUT. NOTE A: Inputs are driven from 0.45 V to 2.4 V. Time intervals are referenced to 2 V if the high level is indicated and 0.8 V if the low level is indicated.
13
Bit 1 = MSB = sign bit and locked in first on the PCM IN or clocked out first on PCM OUT. Bit 8 = LSB = least significant bit and is clocked in last on PCM IN or is clocked out last on PCM OUT. NOTE A: IAll timing parameters referenced to VIH and VIL except tpd7 and tpd8, which reference the high-impedance state.
Bit 1 = MSB = sign bit and locked in first on the PCM IN or clocked out first on PCM OUT. Bit 8 = LSB = least significant bit and is clocked in last on PCM IN or is clocked out last on PCM OUT. NOTE A: All timing parameters referenced to VIH and VIL except tpd7 and tpd8, which reference the high-impedance state.
14
PRINCIPLES OF OPERATION
system reliability and design considerations TCM29C23, TCM129C23 system reliability and design considerations are described in the following paragraphs. latch-up Latch-up is possible in all CMOS devices. It is caused by the firing of a parasitic SCR that is present due to the inherent nature of CMOS. When a latch-up occurs, the device draws excessive amounts of current and will continue to draw heavy current until power is removed. Latch-up can result in permanent damage to the device if supply current to the device is not limited. Even though the TCM29C23 and TCM129C23 are heavily protected against latch-up, it is still possible to cause latch-up under certain conditions in which excess current is forced into or out of one or more terminals. Latch-up can occur when the positive supply voltage drops momentarily below ground, when the negative supply voltage rises momentarily above ground, or possibly if a signal is applied to a terminal after power has been applied but before the ground is connected. This can happen if the device is hot-inserted into a card with the power applied, or if the device is mounted on a card that has an edge connector and the card is hot-inserted into a system with the power on. To help ensure that latch-up does not occur, it is considered good design practice to connect a reverse-biased Schottky diode (with a forward voltage drop of less than or equal to 0.4 V 1N5711 or equivalent) between the power supply and GND (see Figure 7). If it is possible that a TCM29C23- or TCM129C23-equipped card that has an edge connector could be hot-inserted into a powered-up system, it is also important to ensure that the ground edge connector traces are longer than the power and signal traces so that the card ground is always the first to make contact. device power-up sequence Latch-up can also occur if a signal source is connected without the device being properly grounded. A signal applied to one terminal could then find a ground through another signal terminal on the device. To ensure proper operation of the device and as a safeguard against this sort of latch-up, it is recommended that the following power-up sequence always be used: 1. Ensure that no signals are applied to the device before the power-up sequence is complete. 2. Connect GND. 3. Apply VBB (most negative voltage). 4. Apply VCC (most positive voltage). 5. Force a power down condition in the device. 6. Connect clocks. 7. Release the power down condition. 8. Apply FS synchronization pulses. 9. Apply the signal inputs. When powering down the device, this procedure should be followed in the reverse order.
15
PRINCIPLES OF OPERATION
VCC
DGND
VBB
Figure 7. Latch-Up Protection Diode Connection internal sequencing On the transmit channel, digital outputs PCM OUT and TSX are held in the high-impedance state for approximately four frames (500 s) after power up or application of VBB or VCC. After this delay, PCM OUT, TSX, and signaling are functional and occur in the proper time slot. The analog circuits on the transmit side require approximately 60 ms to reach their equilibrium value due to the autozero circuit settling time. Thus valid digital information, such as on/off hook detection, is available almost immediately while analog information is available after some delay. To further enhance system reliability, PCM OUT and TSX are placed in the high-impedance state approximately 20 s after an interruption of CLKX.
16
variable-data-rate timing
Variable-data-rate timing is selected by connecting DCLKR to the bit clock for the receive PCM highway rather than to VBB. It uses master clocks CLKX and CLKR, bit clocks DCLKX and DCLKR, and frame-synchronization clocks FSX and FSR. Variable-data-rate timing allows for a flexible data frequency. The frequency of the bit clocks can be varied from 64 kHz to 4.096 MHz. The bit clocks must be asynchronous. When the FSX/TSXE input is high, PCM data is transmitted from PCM OUT onto the highway on the next eight consecutive positive transitions of DCLKX. Similarly, while the FSR/TSRE input is high, the PCM word is received from the highway by PCM IN on the next eight consecutive negative transitions of DCLKR. The transmitted PCM word will be repeated in all remaining time slots in the frame as long as DCLKX is pulsed and FSX is held high. This feature, which allows the PCM word to be transmitted to the PCM highway more than once per frame if desired, is available only with variable-data-rate timing. Signaling is allowed only in the fixed-data-rate mode because the variable-data-rate mode provides no means with which to specify a signaling frame.
asynchronous operation
In order to avoid crosstalk problems associated with special interrupt circuits, the design includes separate digital-to-analog converters and voltage references on the transmit and receive sides to allow completely independent operation of the two channels. In either timing mode, the master clock, data clock, and time-slot strobe must be synchronized at the beginning of each frame. Specifically, in the variable-rate mode, the falling edge of CLKX must occur within td(FSX) ns after the rise of FSX and the falling edge of DCLKX must occur within tTSDX ns after the rise of FSX. CLKX and DCLKX are synchronized once per frame but may be of different frequencies. The receive channel operates in a similar manner and is completely independent of the transmit timing (see Figure 6). This approach requires the provision of two separate master clocks but avoids the use of a synchronizer, which can cause intermittent data conversion errors.
17
transmit operation
transmit filter The input section provides gain adjustment in the pass band by means of an on-chip uncommitted operational amplifier. The load impedance to ground (ANLG GND) at the amplifier output (GSX) must be greater than 10 k in parallel with less than 50 pF. The input signal on ANLG IN + can be either ac or dc coupled. The input operational amplifier can also be used in the inverting mode or differential amplifier mode. A low-pass antialiasing section is included on the device. This section provides 35-dB attenuation at the sampling frequency. No external components are required to provide the necessary antialiasing function for the switched-capacitor section of the transmit filter. encoding The encoder internally samples the output of the transmit filter and holds each sample on an internal sampleand-hold capacitor. The encoder performs an analog-to-digital conversion on a switched-capacitor array. Digital data representing the sample is transmitted on the first eight data clock bits of the next frame. The autozero circuit corrects for dc offset on the input signal to the encoder using the sign bit averaging technique. The sign bit from the encoder output is long-term averaged and subtracted from the input to the encoder. All dc offset is removed from the encoder input waveform.
receive operation
decoding The serial PCM word is received at PCM IN on the first eight data clock bits of the frame. Digital-to-analog conversion is performed, and the corresponding analog sample is held on an internal sample-and-hold capacitor. This sample is transferred to the receive filter. receive filter The receive section of the filter provides pass-band flatness and stop-band rejection that fulfills both the AT&T D3/D4 specification and CCITT recommendation G.712. The filter contains the required compensation for the (sin x)/x response of such decoders. receive output power amplifiers A balanced output amplifier is provided to allow maximum flexibility in output configuration. Either of the two outputs can be used single ended (i.e., referenced to ANLG GND) to drive single-ended loads. Alternatively, the differential output directly drives a bridged load. The output stage is capable of driving loads as low as 300 single ended to a level of 12 dBm or 600 differentially to a level of 15 dBm. The receive channel transmission level may be adjusted between specified limits by manipulating of the GSR input. GSR is internally connected to an analog gain-setting network. When GSR is connected to PWRO +, the level is minimum. The output transmission level between 0 and 12 dB as GSR is adjusted (with an adjustable resistor) between PWRO + and PWRO . Transmission levels are specified relative to the receive channel output under digital milliwatt conditions (i.e., when the digital input at PCM IN is the eight-code sequence specified in CCITT recommendation G.711).
18
PWRO+
RL
19
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