Fingerprint Scanner (GT-521Fxx) Hookup Guide

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Contributors: bboyho
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Firmware Overview

If you are interested, this section goes just a little further by looking briefly at the command protocol. We will be taking a quick look at the fingerprint scanner's blink example with an Arduino and how the command protocol functions based on the manual.

Verifying the Checksum Value

To verify the check sum for the command packet (command) or response packet (acknowledge), you would add the bytes of the command start codes, device id, parameter, and command/response. Looking at the Arduino blink example, the serial monitor outputs:

FPS - Open
FPS - SEND: "55 AA 01 00 00 00 00 00 01 00 01 01"
FPS - RECV: "55 AA 01 00 00 00 00 00 30 00 30 01"

FPS - LED on
FPS - SEND: "55 AA 01 00 01 00 00 00 12 00 13 01"
FPS - RECV: "55 AA 01 00 00 00 00 00 30 00 30 01"

FPS - LED off
FPS - SEND: "55 AA 01 00 00 00 00 00 12 00 12 01"
FPS - RECV: "55 AA 01 00 00 00 00 00 30 00 30 01"

The example displays the packet structure as a multi-byte item represented as little endian. Breaking down the LED command to turn the LED OFF in hex, it is:

55 AA 01 00 00 00 00 00 12 00 12 01
, where Command Start code1 = 0x55
        Command Start code2 = 0xAA
        Device ID = 0x00 01
        Input parameter = 0x00 00 00 00
        Command Code = 0x00 12

By adding the hex values with a programmer's calculator as stated in the datasheet:

OFFSET[0] + OFFSET[1] + OFFSET[2] + OFFSET[3] + OFFSET[4] + OFFSET[5] + OFFSET[6] + OFFSET[7] + OFFSET[8] + OFFSET[9] = 0x55 +0xAA + 0x01 + 0x00 + 0x00 + 0x00 + 0x00 + 0x00 + 0x12 + 0x00

, we are able to get the same output result as the command packet's check sum:

Checksum = 0x01 12

Since the check sum is read as little endian, the output reads the checksum as "12 01".