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U421 PCB Pin Out


"DIP-like" Connector:

Pin Number U421 Signal
1 PA.0 - Port A bit 0 (stepper motor control)
2 PA.1 - Port A bit 1 (stepper motor control)
3 PA.2 - Port A bit 2 (stepper motor control) (2-wire clock)
4 PA.3 - Port A bit 3 (stepper motor control) (2 wire data)
5 PB.0 - Port B bit 0
6 PB.2 - Port B bit 2
7 PB.4 - Port B bit 4
8 PB.6 - Port B bit 6
9 Ground
10 No Connect
11 No Connect
12 Do not use
13 No Connect
14 +5V (USB)
15 USB D- (already connected to the USB cable)
16 USB D+ (already connected to the USB cable)
17 PB.7 - Port B bit 7
18 PB.5 - Port B bit 5
19 PB.3 - Port B bit 3
20 PB.1 - Port B bit 1
21 PA.7 - Port A bit 7 (or SPI SCK) (stepper motor control)
22 PA.6 - Port A bit 6 (or SPI MISO) (stepper motor control)
23 PA.5 - Port A bit 5 (or SPI MOSI) (stepper motor control)
24 PA.4 - Port A bit 4 (or SPI SS [when in slave mode]) (stepper motor control)

Other Connectors:

There is a 2x6 pad layout on the board that can be used for connection to an AVR (such as an ATtiny) for programming. The 1x2 jumper applies 5V to this header. (Newest U421 board does not have this jumper - the 5V is on ther 2x6 header.) The AVR programmer header follows that of the standard 2x6 AVR programming connector. Connections must be made on the component pads to allow signals out to this header.

The U421-SC3 (available through StrandControl) uses this connection for a way to attach 1-Wire devices. Connections must be made on the component pads to allow signals out to the screw terminal - the 'SC3' is sold with the right connections made.

Power Connection:

The +5V available on pin 14 is the power from the computer's USB port. Up to 100mA can be drawn from this supply to power devices attached to the U421. If more power is needed, an external supply should be used to power additional circuits. Do not attach the +5V from an external supply to this pin. The ground connection should be common to any external power supply ground as well as any target circuits.

Board Layout:

(U421-SC3 screw terminal version of the U421 available through StrandControl) A standard U421 does not have the screw terminal.

Pin 1 on the U421 is located on the lower left, as you look at the board in the orientation above. Pin 24 is the pin on the upper left. The lower row therefore is 1-12, the upper row is 24-13.


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While every effort has been made to make sure that the information posted on this site is correct, the author can not be held liable for any damages whatsoever for losses as a result of the application of this information. Use this information at your own risk.

USBmicro can design your custom and semi-custom USB product. Email about USB design can be directed to " Robert " at usbmicro.com.

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