Platform: atmelavr
=========================================================================
ID              MCU       Frequency Flash  RAM  Name
--------------  --------  --------- ------ ---- -------------------------
gemma           ATTINY85  8MHz      8KB    512B Adafruit Gemma
trinket3        ATTINY85  8MHz      8KB    512B Adafruit Trinket 3V/8MHz
trinket5        ATTINY85  16MHz     8KB    512B Adafruit Trinket 5V/16MHz
digispark-tiny  ATTINY85  16MHz     5.87KB 512B Digispark USB
attiny85        ATTINY85  8MHz      8KB    512B Generic ATtiny85

#define SS   3
#define MOSI 1
#define MISO 0
#define SCK  2

static const uint8_t SDA = 0;
static const uint8_t SCL = 2;

static const uint8_t A0 = 0x80 | 0;
static const uint8_t A1 = 0x80 | 1;
static const uint8_t A2 = 0x80 | 2;
static const uint8_t A3 = 0x80 | 3;

#define PIN_B0  ( 0)
#define PIN_B1  ( 1)
#define PIN_B2  ( 2)
#define PIN_B3  ( 3)
#define PIN_B4  ( 4)
#define PIN_B5  ( 5)

#define PIN_PB0  ( 0)
#define PIN_PB1  ( 1)
#define PIN_PB2  ( 2)
#define PIN_PB3  ( 3)
#define PIN_PB4  ( 4)
#define PIN_PB5  ( 5)

[env:attiny85]
platform = atmelavr
board = attiny85
framework = arduino
upload_protocol = custom
upload_port = /dev/ttyUSB0
upload_speed = 19200
board_build.f_cpu = 8000000L
upload_flags = 
	-C
	$PROJECT_PACKAGES_DIR/framework-arduino-avr-attiny/avrdude.conf
	-p
	$BOARD_MCU
	-P
	$UPLOAD_PORT
	-b
	$UPLOAD_SPEED
	-c
	stk500v1
upload_command = avrdude $UPLOAD_FLAGS -U flash:w:$SOURCE:i

#include <SoftwareSerial.h>

#define JoyStick_LX  A1    // 2  LEFT  LEFT/RIGHT
#define JoyStick_LY  A3    // 3  LEFT  UP/DOWN
#define JoyStick_RX  A2    // 4  RIGHT LEFT/RIGHT
#define JoyStick_RY  A0    // 5  RIGHT UP/DOWN (RESET)
#define PIN_TX       PB0
#define PIN_RX       PB1

SoftwareSerial mySerial(PIN_RX, PIN_TX);

void setup() 
{
	mySerial.begin(9600);
}

void loop()
{
	uint8_t  i;
	uint16_t Lx, Ly, Rx,Ry;
	uint8_t  Lxh, Lxl, Lyh, Lyl, Rxh, Rxl, Ryh, Ryl;

	Lx = analogRead(JoyStick_LX);
	Ly = 1023 - analogRead(JoyStick_LY);
	Rx = analogRead(JoyStick_RX);
	Ry = 1023 - analogRead(JoyStick_RY);
	Lxh = (Lx>>5)|0x00;
	Lxl = (Lx&0x1F)|0x20;
	Lyh = (Ly>>5)|0x40;
	Lyl = (Ly&0x1F)|0x60;
	Rxh = (Rx>>5)|0x80;
	Rxl = (Rx&0x1F)|0xA0;
	Ryh = (Ry>>5)|0xC0;
	Ryl = (Ry&0x1F)|0xE0;
	mySerial.write(Lxh);
	mySerial.write(Lxl);
	mySerial.write(Lyh);
	mySerial.write(Lyl);
	mySerial.write(Rxh);
	mySerial.write(Rxl);
	mySerial.write(Ryh);
	mySerial.write(Ryl);
}

PLATFORM: Atmel AVR (5.1.0) > Generic ATtiny85
HARDWARE: ATTINY85 8MHz, 512B RAM, 8KB Flash
PACKAGES:
 - framework-arduino-avr-attiny @ 1.5.2
 - tool-avrdude @ 1.60300.200527 (6.3.0)
 - toolchain-atmelavr @ 1.70300.191015 (7.3.0)
Dependency Graph
|-- SoftwareSerial @ 1.0.0
RAM:   [==        ]  24.2% (used 124 bytes from 512 bytes)
Flash: [==        ]  23.1% (used 1890 bytes from 8192 bytes)
avrdude: safemode: Fuses OK (E:FF, H:DF, L:E2)

avrdude done.  Thank you.

uint16_t Lx, Ly, Rx, Ry;

void setup() 
{
	Serial1.begin(9600);
	Serial.begin(115200);
	Lx = 0;
	Ly = 0;
	Rx = 0;
	Ry = 0;
}

void loop()
{
	uint16_t ival;
	uint16_t sLx, sLy, sRx, sRy;

	while (Serial1.available()) {
		ival = Serial1.read();
		switch(ival&0xE0) {
			case 0x00: // 0b000xxxxx
				sLx = ((ival & 0x1F)<<5) | 0x8000;
				 break;
			case 0x20: // 0b001xxxxx
				if (sLx & 0x8000) Lx = (sLx | (ival & 0x1F)) & 0x03FF;
				sLx = 0x0;
				break;
			case 0x40: // 0b010xxxxx
				sLy = ((ival & 0x1F)<<5) | 0x8000;
				 break;
			case 0x60: // 0b011xxxxx
				if (sLy & 0x8000) Ly = (sLy | (ival & 0x1F)) & 0x03FF;
				sLy = 0x0;
				break;
			case 0x80: // 0b100xxxxx
				sRx = ((ival & 0x1F)<<5) | 0x8000;
				 break;
			case 0xA0: // 0b101xxxxx
				if (sRx & 0x8000) Rx = (sRx | (ival & 0x1F)) & 0x03FF;
				sRx = 0x0;
				break;
			case 0xC0: // 0b110xxxxx
				sRy = ((ival & 0x1F)<<5) | 0x8000;
				 break;
			case 0xE0: // 0b111xxxxx
				if (sRy & 0x8000) Ry = (sRy | (ival & 0x1F)) & 0x03FF;
				sRy = 0x0;
				break;
		}
	}
	Serial.printf("%04d %04d %04d %04d\n",Lx,Ly,Rx,Ry);
	delay(1000);
}