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Texas Instruments Deutschland introduced in 1977 with the TMS3878NS a compact and highly integrated solution to display the frequency of the selected station in multiband radios with analog tuners. The first known application was with Grundig’s Satellit 3000 world-band receiver, manufactured by Grundig in Portugal and using a red 4½ digit LED display to show the frequency. The closely related TMS3707NC was introduced in 1979 and is known from various Philips AM/FM car radios like the Coupé Digital 890 and 894 (22AC890, 22AC894) featuring a 3½-digit green LED readout display, three band LW/MW/FM stereo radio and 2x6 Watt amplifier.

The TMS3878 combines a 5-digit BCD (Binary Coded Decimal) counter with Intermediate Frequency (IF) correction, 5-digit display register, digit multiplexer, leading-zero suppression, 7-segment decoder and display drivers in a 24-pin package and operates from a single 5 Volt supply. To reduce both power consumption and switching noise on the power supply, the TMS3878 is operated at a clock frequency of only 320 kHz and switching the individual segments of the activated digit on and off in a staggered timing.

The Intermediate Frequency correction is selectable in FM Mode between 10.6 MHz, 10.7 MHz and 10.8 MHz, in SW Mode the IF correction is set to 2.0 MHz and in AM Mode to 0.46 MHz.

Based on the selected Mode and IF, the TMS3878 will preload its 5-digit BCD counter before each measurement cycle with corresponding values, e.g. 989.30 for 10.7 MHz and thus resulting in a displayed value of the input frequency minus IF.

All internal timing signals are derived from the 320 kHz clock frequency, which divided by 32 results in precise 0.1 ms intervals. One measurement cycle takes 256 ms, equivalent to about four measurements per second. The actual frequency measured by the TMS3878 is not the antenna frequency (f_ANT) of the radio but the oscillator frequency of the analog tuner (f_OSC = f_ANT + IF) divided by either 512 (FM Mode), 128 (SW Mode) or 16 (AM Mode), resulting in a frequency range between 37 kHz and 232 kHz in a typical tri-band radio. At the beginning of each measurement cycle the 5-digit BCD counter is preloaded with the desired IF correction value and the counting gate is opened for either 51.2 ms (FM Mode), 128 ms (SW Mode) or 16 ms (AM Mode). The beginning of each measurement cycle is signaled with a short pulse (0.8 ms in FM and SW Mode and 0.1ms in AM Mode) to reset the prescaler connected to the pulse input of the TMS3878.

During display scanning each digit, starting with the leftmost digit D1, is activated for 3.3 ms with an inter-digit blanking of 0.1 ms, resulting in 17 ms for five digits or about 60 scans per second.

The TMS3878 features leading-zero suppression in FM, SW and AM Mode:

• In FM Mode the decimal point is output at Digit 3, resulting in a display of e.g. _87.50 or 103.45 (MHz) • In SW Mode the decimal point is output at Digit 2, resulting in a display of e.g. _1.600 or 30.000 (MHz) • In AM Mode the decimal point is output at Digit 2, resulting in a display of e.g. _0.150 or _1.620 (MHz)

The digit and segment outputs of the TMS3878 are active high with PMOS transistors conducting to the positive power rail V_SS with the segment outputs capable of driving small, common cathode LED displays directly. For larger displays requiring more than 5 mA current per segment, external drivers are necessary.

Type	Products	Function
TMS3878NS	Grundig Satellit 3000, 3400	Using 4½-digit LED display
TMS3707NC	Philips Coupé Digital 890, 894	Using 3½-digit LED display

Note: Learn more about the MOS Numbering System used by Texas Instruments' early MOS Chips.

Item	Min	Typ	Max	Unit	Comments
VSS		0		V
VDD	-5.5	-5.0	-4.5	V
IDD		-4.5		mA
VIH	-1.5		0.3	V	High-level Input Voltage CK, PI, MODE (B1, B2, BIH, BIL)
VIL	VDD		-3.5	V	Low-level Input Voltage CK, PI, MODE (B1, B2, BIH, BIL)
II		-0.6		mA	Input Current CK, PI (VI= -5 V)
II		-0.3		mA	Input Current MODE (VI= -5 V)
VOH		-1.8		V	High-level Output Voltage SEG, DP, GATE (IOH= -2 mA)
VOH		-1.8		V	High-level Output Voltage DIGITS (IOH= -1 mA)
fOSC		320		kHz	Oscillator Frequency Ø (Crystal controlled time base)
fPI	0		750	kHz	Input Frequency PI

FREQUENCY COUNTER

Digital frequency counters usually count the number of pulses during a specific period of time, known as gate time. With a gate time of 1 second, the counter value would match at the end of the gate time with the input frequency in cycles per second or hertz (Hz). In a typical implementation of a frequency counter the gate time is derived from a precise clock oscillator called time-base, the counter reset to zero at the beginning of the gate time and the counter value transferred to a display at the end of the gate time.

In applications that demand a higher measuring rate and accept a reduced resolution, the gate time can be shortened. In some cases prescalers are used to divide high input frequencies into lower frequencies that the counter can resolve.

Texas Instruments is using with the TMS3878 Five Decade Counter both approaches and adjusting both the gate time and external prescaler based on the B1 and B2 Mode Input Pins:

Mode Pins B2 B1	Mode	Gate time	Prescaler
0   0	AM	16 ms	:16
0   1	SW	128 ms	:128
1   0	HOLD	0 ms
1   1	FM	51.2 ms	:512

While the gate time varies greatly between 16 ms and 128 ms, is the measurement cycle constant with 256 ms or about 4 display updates per second. The gate time and prescaler combinations selected for AM Mode and FM Mode are chosen in a way to have the tuner frequencies matching the counter value of a 5-digit counter.

Example AM Mode, 1620 kHz:

• Input frequency TMS3878 = 1620 kHz / 16 = 101.25 kHz • Pulses during gate time = 16 ms * 101.25 kHz = 1620

Example SW Mode, 26.4 MHz:

• Input frequency TMS3878 = 26.4 MHz / 128 = 206.25 kHz • Pulses during gate time = 128 ms * 206.25 kHz = 26400

Example FM Mode, 103.45 MHz:

• Input frequency TMS3878 = 103.45 MHz / 512 = 202.0508 kHz • Pulses during gate time = 51.2 ms * 202.0508 kHz = 10345

The above examples are used to explain the principle of the frequency counter, the TMS3878 is always modifying the display value with its integrated Intermediate Frequency (IF) correction.

The gate time of the TMS3878 is generated from its clock input Ø which is specified with 320 kHz and typically generated with an external crystal oscillator running at 5.12 MHz and divided by 16 with an external 4-stage binary counter.

INTERMEDIATE FREQUENCY CORRECTION

In its intended application, the actual frequency measured by the TMS3878 is not the antenna frequency (f_ANT) of the radio, but the oscillator frequency of the analog tuner (f_OSC = f_ANT + IF) divided by either 512 (FM Mode), 128 (SW Mode) or 16 (AM Mode).

To display the value of the antenna frequency, the Intermediate Frequency must be subtracted from the oscillator frequency. The TMS3878 allows in FM Mode three different IF settings between 10.6 MHz and 10.8 MHz, while the SW and AM Modes are using fixed settings of 2.0 MHz and 0.46 MHz, respectively.

The subtraction of the desired IF value is accomplished with preloading the 5-digit counter to a negative value equivalent to the selected IF at the beginning of a measurement cycle:

Mode Pins BIH BIL	Mode	IF	Preload Value
0   0	FM	10.7 MHz	989.30
0   1	FM	10.8 MHz	989.20
1   X	FM	10.6 MHz	989.40
X   X	SW	2.00 MHz	98.000
X   X	AM	0.46 MHz	99.540

Example AM Mode, 1620 kHz, IF = 460 kHz:

• fOSC= fANT + IF = 2080 kHz • Input frequency TMS3878 = 2,080 kHz / 16 = 130 kHz • Pulses during gate time = 16 ms * 130 kHz = 2080 • Counter preload value = 99.540 • Counter value = 99540 + 2080 = 101620

Counter value 101620 with the leftmost digit lost for 01620, displayed with leading-zero suppression and the decimal point placed at the second digit from the left as 1.620 (MHz).

Example SW Mode, 26.4 MHz, IF = 2.0 MHz:

• fOSC= fANT + IF = 28.4 MHz • Input frequency TMS3878 = 28.4 MHz / 128 = 221.875 kHz • Pulses during gate time = 128 ms * 221.875 kHz = 28400 • Counter preload value = 98.000 • Counter value = 98000 + 28400 = 126400

Counter value 126400 with the leftmost digit lost for 26400, displayed with the decimal point placed at the second digit from the left as 26.400 (MHz).

Example FM Mode, 103.45 MHz, IF = 10.7 MHz:

• fOSC= fANT + IF = 114.15 MHz • Input frequency TMS3878 = 114.15 MHz / 512 = 222.9492 kHz • Pulses during gate time = 51.2 ms * 222.9492 kHz = 11415 • Counter preload value = 989.30 • Counter value = 98930 + 11415 = 110345

Counter value 110345 with the leftmost digit lost for 10345, displayed with the decimal point placed at the third digit from the left as 103.45 (MHz).

The actual implementation of the TMS3878 uses the logic level of its Pulse Input as lowest bit for the display, therefore the last Flip-Flop of the prescaler needs to be set to zero at the beginning of the measurement cycle, too. Texas Instruments did not incorporate additional logic to preload the last bit of the prescaler with the corresponding bit of the Intermediate Frequency and consequently only even IF correction values are feasibly and resulting in a rather unusual IF of 460 kHz for AM instead the more common 455 kHz.

Tri-band car stereos sold in Continental Europe in the Eighties usually supported the following frequency ranges in each band:

• L/GO (Langwelle, Long Wave, Grande Onde) 150 kHz – 260 kHz • M/PO (Mittelwelle, Medium Wave, Petite Onde) 512 kHz – 1620 kHz • U/FM (Ultrakurzwelle, Ultra-short Wave, Fréquence Modulation) 87.5 MHz – 104 MHz

Together with the :16 prescaler and 460 kHz IF for the two lower frequency bands and the :512 prescaler and 10.8 MHz IF for the higher frequency band, the resulting input frequencies on the Pulse Input of the TMS3878 cover a range between 38.125 kHz and 224.219 kHz in a typical tri-band radio.

Grundig’s Satellit 3000 and 3400 multiband receivers cover 13 bands with the following frequencies:

• LW 145 kHz – 420 kHz • MW 510 kHz – 1620 kHz • K1, K2 1.6 MHz - 5.2 MHz • K3-K10 5.0 MHz - 30 MHz • FM 87.5 MHz – 108 MHz

The highest frequency on the Pulse Input of the TMS3878 can be found in the K10 band at 30 MHz with the :128 prescaler and 2.0 MHz IF calculated with 250 kHz.

DISPLAY REGISTER

The 5-digit display register of the TMS3878 is loaded at the end of each gate time with the current value of the 5-digit counter to allow a flicker-free reading of the selected station frequency on the 3½-digit or 4½ digit LED display. While the duration of each measurement cycle is constant with 256 ms, varies the gate time between 16 ms in AM Mode, 51.2 ms in FM Mode and 128 ms in SW Mode. Additionally are, depending from the selected Mode, different Intermediate Frequency values preloaded into the 5-digit counter and we observed the following sequence with a TMS3707NC Device-under-Test (DUT):

• 1: Start of Measurement Cycle: Short Gate Output pulse to reset :16 prescaler • 2: Preload 5-digit counter with desired IF correction value based on Mode Pins • 3: Load gate time counter with desired gate time value • 4: Open gate time and count pulses on Pulse Input Pin. Pin Level is LSB of 5-digit counter • 5: Close gate time and transfer 5-digit counter value into 5-digit display register • 6: End of Measurement Cycle

To verify the transfer of the 5-digit counter to the 5-digit display register at the end of the gate time and not at the end of the measurement cycle, we developed a simple test setup:

Applying a frequency to the Pulse Input that resulted in a flickering of the last digit of the LED display between 0 and 1, we connected two channels of a Digital Oscilloscope (DSO) to Segment A and Segment B Outputs of the TMS3707NC DUT, qualified with its Digit 5 (right-most) Output. While Segment A is only active with the 7-Segment pattern of a "0", is Segment B active with both a "0" and "1". Connecting another channel of the DSO to the Gate Output Pin of the TMS3707NC DUT and switching the Mode Pins through AM Mode, FM Mode, and SW Mode, confirmed that the display is updated after 16 ms, 51.2 ms, and 128 ms, respectively.

DISPLAY SCANNING

The 5-digit display register is continuously scanned in D1 → D2 → D3 → D4 → D5 → D1 sequence and transmitted to the external LED display through the integrated BCD to 7-Segment decoder and display drivers.

Within each digit activation time of 3.3 ms the segments are activated for only 2.4 ms in a staggered timing. Segment A is switched on 0.1 ms after the selected digit is activated and switched off at 2.5 ms, Segment B is switched on at 0.2 ms and switched off at 2.6 ms and so forth. This novel approach slows the inrush current peak of the power supply dramatically when turning on a larger number of segments. Assuming 30 mA current per LED, turning on all segments at once would create a 210 mA inrush current while with the staggered timing the LED current increases slowly in 30 mA steps every 0.1 ms hence requiring much smaller buffer capacitors for the power supply and creating less switching noise.

Turning the segments a few hundred microseconds before the corresponding digits off, allows the use of thyristors (Silicon Controlled Rectifiers or SCRs) as digit drivers, too.

With five digits, a digit time of 3.3 ms and an inter-digit blanking time of 0.1 ms, each display scanning cycle takes 17 ms or about 60 scans per second. Please notice that display scanning with 17 ms cycle time is slightly asynchronous to the measurement cycle with 256 ms, 15 display scan cycles complete in 255 ms.

BCD TO SEVEN-SEGMENT DECODER

The TMS3878 integrates a BCD to Seven Segment decoder with the following segment patterns:

Type	Number Fonts
TMS3878NS

MODE/IF SELECTION INPUT PINS CHARACTERISTICS

The four Mode and IF Selection Pins of the TMS3878 are using positive logic, voltage levels close to the positive voltage (V_SS) are interpreted as a logical "1" and voltage levels close to the negative voltage (V_DD) are interpreted as a logical "0". Internal pull-up resistors of about 15 kOhm to V_SS results in open input pins (not connected or n.c.) reading a logical "1". Threshold voltage of the input pins was measured on the device under test between 2.3 V and 2.5 V.

CLOCK/PULSE INPUT PINS CHARACTERISTICS

The Clock Input Pin of the TMS3878 is used to generate all internal timing signals like time-base, measurement time, display scanning, prescaler reset and typically connected to a precise 320 kHz reference frequency. The TMS3878 uses the positive edge of the clock signal and utilizes an internal pull-up resistor of about 7.5 kOhm to V_SS. Threshold voltage of the Clock Input Pin was measured on the TMS3707NC device under test between 2.3 V and 2.5 V.

The Pulse Input Pin of the TMS3878 serves two purposes. Being connected directly to the output of the :16 prescaler, the level of the Pulse Input Pin is treated as the lowest bit of the 5-decade counter and its positive edge is used to toggle the next bits of the 5-decade counter. The Pulse Input Pin utilizes an internal pull-up resistor of about 7.5 kOhm to V_SS. Threshold voltage of the Clock Input Pin was measured on the TMS3707NC Device-under-Test (DUT) between 2.3 V and 2.5 V.

GATE OUTPUT PIN CHARACTERISTICS

Main purpose of the Gate Output Pin is to signal the beginning of a measurement cycle of the frequency counter and therefore resetting the external :16 prescaler.

The TMS3878 uses negative logic for the Gate Output PIN, generating a short pulse to V_DD at the beginning of each measurement cycle. The pulse duration varies between 32 clock pulses (0.1 ms at 320 kHz) in AM Mode and 256 clock pulses (0.8 ms at 320 kHz) in SW and FM Mode.

The Gate Output Pin uses an open-drain PMOS transistor having a nominally 600 Ohm on-state impedance (measured at V_OH = -2.5 V), additionally did we measure a weak pull-down transistor to V_DD of about 70 kOhm, rendering its logical low output voltage compatible to CMOS devices.

DISPLAY DRIVER OUTPUT PINS CHARACTERISTICS

The TMS3878 used for the serially scanned 5 Digit Outputs D1 to D5, the 7-Segment Outputs SA to SG, and the Decimal Point Output DP positive logic. The active digit time is signaled with a logical "1" as is an activated segment or decimal point.

The Digit Output Pins D1 to D5 are using an open-drain PMOS transistor having a nominally 1,200 Ohm on-state impedance (measured at V_OH = -2.5 V), additionally did we measure on the TMS3707NC DUT a weak pull-down transistor to V_DD of about 70 kOhm.

The Segment Outputs SA to SG and Decimal Point Output DP are using an open-drain PMOS transistor having a nominally 600 Ohm on-state impedance (measured at V_OH = -2.5 V), additionally did we measure a on the TMS3707NC DUT weak pull-down transistor to V_DD of about 70 kOhm.

Adding a weak-pull down transistor to V_DD to all Display Driver Outputs allows connecting different types of external display drivers to the TMS3878.

COMMON CATHODE DISPLAYS

The TMS3878 can be used for small portable, battery-operated radios with calculator-style 7-segment displays in common cathode (CC) configuration. The duty cycle of the Digit Outputs D1 to D5 is about 20%, typical calculator displays with bubble lenses would require about 5 mA segment current for good indoor readability of the selected station. The resulting digit current with all segments and the decimal point illuminated equals to 40 mA, well within the 100 mA specification of the SN75497 Digit Driver Chip. The seven segments and decimal point of the calculator display can be directly driven with the Segment Outputs SA to SG and the Decimal Output DP of the TMS3878, its nominally 600 Ohm on-state impedance limits the output currents (assuming V_DD = -5 V, TIL393-6 display with V_F = 1.7 V and SN75497 with V_OL = 0.2 V) to about 5 mA.

COMMON ANODE DISPLAYS

Larger LED displays used in car stereos are operated around 30 mA peak current per segment to allow perfect readability even under difficult ambient light conditions. Using a multiplexed 7-segment display in common anode (CA) configuration allows for a cost-optimized solution using external digit and segment drivers with the TMS3878. The SN75498 Digit Driver Chip is used for the seven segments A to G and decimal point and provides nine drivers with a maximum current of 100 mA per channel, leaving one of the drivers unused. With all segments and the decimal point illuminated, the peak current of the activated digit equals to 240 mA, well above the capability of the Digit Output Pins D1 to D5 with a nominally 1,200 Ohm on-state impedance. Comparing the schematics of various TMS3838/TMS3707 applications shows two very efficient solutions.

Grundig chose with the Satellit 3000 and 3400 multiband receivers an approach with logic-level thyristors (Silicon Controlled Rectifiers or SCRs) using less than 0.2 mA gate trigger current. The selected Philips BT169 features a short turn-off time of about 0.1 ms, well below the 1 ms “segment off” time of the TMS3838/TMS3707 devices.

Philips is using a bipolar Darlington NPN transistor operated as an emitter follower and providing the necessary current amplification of about 250 (assuming V_DD = -5 V, MAN4510A display with V_F = 2.5V, BC517 Darlington BJT V_BE = 1.3V and SN75498 with V_OL = 0.2 V).

EXTERNAL LOGIC

Both displaying the frequency of an FM station with a 4½-digit value and displaying the frequency of an AM station in MHz are unusual in Continental Europe and Philips consequently added additional logic to the circuitry of their Coupé Digital 890 and 894 car radios. Both the digit multiplexers for the 3½-digit LED display and the logic for external leading-zero suppression of the TMS3707NC outputs are realized with two CMOS chips and a few diodes and passive components, resulting the following display output:

Mode	TMS3707NC Output	Radio Display
FM	_87.50	_87.5 (MHz)
FM	103.45	103.4 (MHz)
AM	_0.150	150 (kHz)
AM	_1.620	1620 (kHz)

Pin	IO	Function	Pin	IO	Function
1	O	Digit Driver 3	24	V	Common Voltage VSS
2	O	Digit Driver 4	23	O	Digit Driver 2
3	O	Digit Driver 5 (LSD)	22	O	Digit Driver 1 (MSD)
4	I	Clock Input	21	I	IF Select Input BIL
5	O	Segment Driver DP	20	I	IF Select Input BIH
6	O	Segment Driver G	19	O	Gate Output
7	O	Segment Driver F	18	I	Pulse Input
8	O	Segment Driver E	17	I	Mode Input B1
9	O	Segment Driver D	16	I	Mode Input B2
10	O	Segment Driver C	15		Not Connected
11	O	Segment Driver B	14		Not Connected
12	O	Segment Driver A	13	V	Negative Voltage VDD

If you have additions to the above datasheet please email: joerg@datamath.org.

© Joerg Woerner, April 28, 2024. No reprints without written permission.