working version with SPI w/o blinking

841-tinycore-micronucleus-bme280-spi-test/Makefile

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+# Arduino Make file. Refer to https://github.com/sudar/Arduino-Makefile
+
+# attiny841:
+#   BOARD_TAG          = attinyx41
+#   BOARD_SUB          = 841
+# attiny861:
+#   BOARD_TAG          = attinyx61
+#   BOARD_SUB          = 861
+# attiny85:
+#   BOARD_TAG          = attinyx5
+#   BOARD_SUB          = 85
+# attiny84:
+#   BOARD_TAG          = attinyx4
+#   BOARD_SUB          = 84
+
+ARDUINO_VERSION = 10810
+
+PROJECT_DIR              = $(shell pwd)
+BOARD_TAG                = attinyx41
+BOARD_SUB                = 841
+ARDUINO_DIR              = /usr/local/arduino
+ARDMK_DIR                = /usr/local/arduino/Arduino-Makefile
+MONITOR_PORT             = /dev/ttyACM0
+ISP_PORT                 = /dev/ttyACM0
+AVRDUDE                  = /usr/local/bin/avrdude
+ARDUINO_LIBS             = SPI TinyBME280
+ARDUINO_SKETCHBOOK       = .
+
+# mk stuff
+ALTERNATE_CORE           = ATTinyCore
+F_CPU                    = 8000000L
+MONITOR_BAUDRATE         = 115200
+AVRDUDE_ARD_PROGRAMMER   = stk500v2
+AVRDUDE_ARD_BAUDRATE     = 9600
+AVR_TOOLS_DIR            = /usr/local/avr
+AVRDUDE_CONF             = /usr/local/etc/avrdude.conf
+
+# Micronucleus
+MNINST                   = sudo micronucleus
+
+# compiler stuff
+CFLAGS_STD               = -std=gnu11
+CXXFLAGS_STD             = -std=gnu++11
+CXXFLAGS               + = -pedantic -Wall -Wextra
+CURRENT_DIR              = $(shell pwd)
+
+# keep this!
+include $(ARDMK_DIR)/Arduino.mk
+
+# install using micronucleus
+install: $(TARGET_HEX) verify_size
+	@echo "########### Press RESET on the Nanite! ##############"
+	$(MNINST) $(TARGET_HEX)

841-tinycore-micronucleus-bme280-spi-test/README.md

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+# Usable Pins
+
+```
+PIN_A0  (10)
+PIN_A1  ( 9)
+PIN_A2  ( 8)
+PIN_A3  ( 7)
+PIN_A4  ( 6)
+PIN_A5  ( 5)
+PIN_A6  ( 4)
+PIN_A7  ( 3)
+PIN_B0  ( 0)
+PIN_B1  ( 1)
+PIN_B2  ( 2)
+PIN_B3  (11)  /* RESET */
+LED_BUILTIN (2)
+```

841-tinycore-micronucleus-bme280-spi-test/SensorSerial.ino

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+/* -*-c++-*-
+  Blink
+  Turns on an LED on for one second, then off for one second, repeatedly.
+  This example code is in the public domain.
+ */
+
+/*
+  ./hardware/ATTinyCore/avr/cores/tinymodern/core_pins.h
+  Caution: reverse Pinout: PA3 => Pin 3
+
+  Nanite841 Pinout from above:
+
+              button
+SDO/MISO PA5          PA6 SDI/MOSI
+     SCK PA4          PA7
+     CS  PA3          PB2
+     RX0 PA2          PB2
+     TX0 PA1          PB1
+         PA0          PB0
+         GND          VCC
+               usb
+
+  Bosch BME280 Breakout from above pin header left:
+
+    VCC
+    GND
+    SCL  => SCK
+    SDI  => MOSI
+    CSB  => PA3/3
+    SDO  => MISO
+  
+ */
+
+#define TINY_BME280_SPI
+#include <TinyBME280.h>
+
+#define LED LED_BUILTIN
+#define CS  3
+
+tiny::BME280 sensor;
+uint32_t pres, humidity;
+int32_t  temp;
+
+
+void print_measurements() {
+  temp     = sensor.readFixedTempC();
+  humidity = sensor.readFixedHumidity();
+  pres     = sensor.readFixedPressure();
+
+  Serial.print("     Temperature: ");
+  Serial.print(temp / 100.0);
+  Serial.println(" Grad C");
+  Serial.print("        Pressure: ");
+  Serial.print(pres / 100.0);
+  Serial.println(" hPa");
+  Serial.print("        Humidity: ");
+  Serial.print(humidity / 1000.0);
+  Serial.println(" %");
+  Serial.println();
+}
+
+void halt() {
+  while(1);
+}
+
+void setup() {
+  pinMode(LED, OUTPUT);
+  Serial.begin(115200);
+  Serial.println("init");
+  howlong = 500;
+  if(sensor.beginSPI(CS) == false) {
+    Serial.println("Sensor BME280 connect failed, check wiring!");
+    halt();
+  }
+  
+}
+
+void loop() {
+  print_measurements();
+  delay(2000);
+}
+

841-tinycore-micronucleus-bme280-spi-test/hardware

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+/usr/local/arduino/hardware

841-tinycore-micronucleus-bme280-spi-test/libraries/SPI

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+/usr/local/arduino/hardware/ATTinyCore/avr/libraries/SPI