PCB Design

In order to create a functioning product, we need to integrate digital clock IC with the PCB. The PCB provides external support for the IC chip. It includes:

2 different power supplies: a 9V battery and a 2.0 USB connection
Push-button switches for reset and setting mode
Crystal reasoning at 32.768kHz
External Crystal Oscillator
Six 7-Segment LED Displays
Slide switches for selection of power, clock selection, external clock selection, mode selection

The biggest challenge was operating the LED displays at 1.8V. Since our power supply is 1.2V we had to use level shifters and one more voltage regulator. Operation with the 9V battery and the signal from the internal oscillator is a standalone operation. We designed the PCB using.Diptrace Software. The dimension of the board is 9.35 x 7.26 cm, about the size of a small book.

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Fig. 23: PCB Schematic

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Fig. 24: PCB Layout

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Fig. 25: PCB Layout with copper pour

Power Circuitry

The PCB has two different power sources, a portable 9V battery through the jack connector and a 5V 2.0 USB connector. This way it can operate both standalone and plugged to an external power source. The 9V battery also has a 9 - 5V voltage regulator. After the source is selected via a slide switch, we include a 5 - 1.2V voltage regulator for the Vdd and a 5 - 1.8V voltage regulator for the level shifters. Decoupling capacitances are placed appropriately.

Fig. 26: Power Circuitry

IC PIN Connection

The core component of this project is the IC pin that MOSIS manufactured with a DIP-28 pin package. We added a test-point next to every pin to aid the testing process.

Fig. 27: Pin connectivity

Pin#	Pin Name	Pin#	Pin Name
1	V_DD	15	GND
2	HRS_CLK	16	EN_0
3	MIN_CLK	17	EN_1
4	GL_RES	18	EN_2
5	ADV_HRS	19	EN_3
6	ADV_MIN	20	EN_4
7	MODE_SEL	21	EN_5
8	1Hz_TEST	22	A
9	EXT_CLK	23	B
10	CLK_SEL	24	C
11	NC	25	D
12	NC	26	E
13	XIN	27	F
14	XOUT	28	G

Table 1: Pin Configuration

Push-Button & Slide Switches

The push-button and slide switch circuitry is shown below. Normally closed configuration is utilised for push-buttons.

Fig. 28: Push-button & Slide switches

Oscillator Circuit

On the board we have placed two oscillating sources. A crystal which is connected to the internal chip oscillator and an external crystal oscillator just in case our crystal was not working properly. Both resonate at 32.768kHz. The external one is connected to a shunt switch that chooses between that one and the external frequency generator.

Fig. 29: Crystal & External Crystal Oscillator

7-Segment LED Display circuitry

The LED circuitry was the most challenging part of the PCB. We have 7 segment display signals A-G and 6 enable signals EN0-EN5.

7-Segments

The LED displays operate at 1.8V. We get that 1.8V from the power supply using an LDO. Then we use 2 level shifters with 4-inputs each. These take as inputs the 7-segment signals A-G at 1.2V and output the same signals at 1.8V.

Enable Signals

For the enable signals, we first use a 6-input schmidt trigger to invert them because we will use common cathode LEDs. The schimdt trigger's outputs are fed to 2 more level shifters that shift 1.2 to 1.8V and then outputs are connected to LED common ground pin.

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Fig. 30: PCB LED circuitry
After verifying schematic & layout of the PCB we generated the gerber files and sent them to PCB minions. Below we also provide a 3D model of the PCB along with its components.

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Fig. 31: PCB Product

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Fig. 32: PCB Product

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Fig. 33: PCB Product with components

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Fig. 34: PCB Product with components