Thanks for the response Max 
Here’s my code:
#include <Arduino.h>
#define PN PIN_A3
void setup() {
pinMode(PN, OUTPUT);
digitalWrite(PN, HIGH);
//Serial.begin(9600);
}
void loop() {
// put your main code here, to run repeatedly:
digitalWrite(PN, HIGH);
//Serial.println("On");
delay(1000);
//Serial.println("Off");
digitalWrite(PN, LOW);
delay(1000);
}
So, pretty simple I think 
The commented out Serial lines should output the text on PB2, the default serial transmit pin, but they don’t work either, wheras I can see the A3 pin wobbling up and down and the serial data being sent to PB2 when the code is compiled and uploaded from the Arduino IDE :-S
Given the simplicity of the code, I assume that the different MegaTinyCore version wouldn’t affect something as fundamental as this?
The jtag2updi package I am using was downloaded from ElTangas/jtag2updi and compliled in the Arduon IDE and uploaded to my Arduino Uno, and it works fine from both PlatformIO and the Arduino IDE.
The platformio.ini for the project on PlatformIO looks like this:
[env:ATtiny1616]
platform = atmelmegaavr
board = ATtiny1616
framework = arduino
board_build.f_cpu = 8000000L
upload_protocol = custom
upload_port = COM4
upload_speed = 19200
upload_flags =
-C
C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf
-p
$BOARD_MCU
-P
$UPLOAD_PORT
-b
$UPLOAD_SPEED
-c
jtag2updi
-u
-e
upload_command = avrdude $UPLOAD_FLAGS -U flash:w:$SOURCE:i
Just to try to match things, I have also changed the board_build.f_cpu to 20000000, as it is in the Arduino IDE, but that doesn’t make any difference either. There are other settings in the Arduino IDE for the ATtiny1616, but I have no idea how to replicate these :-S
The avrdude.conf file is the one which is provided with jtag2updi, although it was generating some errors about possibly incorrect address lines for members of the ATtiny family, which I think I fixed properly :-S However a compile still yields warnings about other issues, although I have ignored these for time being as they don’t seem to relate to the 1616.
The issue in avrdude.conf seemed to be down to the definition for the ATtiny43U, which seemed to be inherited by other Atiny devices, yielding the following errors:
avrdude warning: ATmega329's flash writepage misses a necessary address bit a13 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:5828]
avrdude warning: ATmega649's flash writepage misses a necessary address bit a14 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:6054]
avrdude warning: AT90PWM2's eeprom writepage misses a necessary address bit a8 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:9244]
avrdude warning: ATtiny44's eeprom writepage misses a necessary address bit a7 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11102]
avrdude warning: ATtiny84's eeprom writepage misses a necessary address bit a8 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11286]
avrdude warning: a4 would normally be expected to be a5 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: a3 would normally be expected to be a4 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: a2 would normally be expected to be a3 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: a1 would normally be expected to be a2 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: a0 would normally be expected to be a1 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: too few opcode bits in instruction [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11571]
avrdude warning: ATtiny43u's eeprom read misses a necessary address bit a5 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11590]
avrdude warning: ATmega32U4's eeprom writepage misses a necessary address bit a2 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11776]
avrdude warning: AT90USB1286's eeprom writepage misses a necessary address bit a11 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:12171]
avrdude warning: ATtiny1634's flash writepage misses a necessary address bit a5 [C:\Users\xxxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:14927]
So I checked the file over and compared various entries, then fixed it by changing line 11571 from (11570 included for context):
read = "1 0 1 0 0 0 0 0 0 0 0 x x x x x",
"0 0 a4 a3 a2 a1 a0 o o o o o o o o";
to:
read = "1 0 1 0 0 0 0 0 0 0 0 x x x x x",
"0 0 a5 a4 a3 a2 a1 a0 o o o o o o o o";
Which got rid of all those errors about the ATtinys, so now what I get in terms of error messages is:
avrdude error: "safemode" feature no longer supported
avrdude warning: ATmega329's flash writepage misses a necessary address bit a13 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:5828]
avrdude warning: ATmega649's flash writepage misses a necessary address bit a14 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:6054]
avrdude warning: AT90PWM2's eeprom writepage misses a necessary address bit a8 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:9244]
avrdude warning: ATtiny44's eeprom writepage misses a necessary address bit a7 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11102]
avrdude warning: ATtiny84's eeprom writepage misses a necessary address bit a8 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11286]
avrdude warning: ATmega32U4's eeprom writepage misses a necessary address bit a2 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:11776]
avrdude warning: AT90USB1286's eeprom writepage misses a necessary address bit a11 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:12171]
avrdude warning: ATtiny1634's flash writepage misses a necessary address bit a5 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:14927]
avrdude warning: mcuid -1 for ATtiny402w is out of range [0..2039], use a free number >= 372 [C:\Users\xxxxx\OneDrive\PlatformIO\Projects\ATTiny1616 Test1\src\jtag2updi\avrdude.conf:15542]
When I compile I get the following output:
Processing ATtiny1616 (platform: atmelmegaavr; board: ATtiny1616; framework: arduino)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Verbose mode can be enabled via `-v, --verbose` option
CONFIGURATION: https://docs.platformio.org/page/boards/atmelmegaavr/ATtiny1616.html
PLATFORM: Atmel megaAVR (1.9.0) > ATtiny1616
HARDWARE: ATTINY1616 16MHz, 2KB RAM, 16KB Flash
PACKAGES:
- framework-arduino-megaavr-megatinycore @ 2.6.7
- tool-avrdude @ 1.70100.0 (7.1.0)
- toolchain-atmelavr @ 3.70300.220127 (7.3.0)
LDF: Library Dependency Finder -> https://bit.ly/configure-pio-ldf
LDF Modes: Finder ~ chain, Compatibility ~ soft
Found 15 compatible libraries
Scanning dependencies...
No dependencies
Building in release mode
Compiling .pio\build\ATtiny1616\src\main.cpp.o
Linking .pio\build\ATtiny1616\firmware.elf
Checking size .pio\build\ATtiny1616\firmware.elf
Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
RAM: [==== ] 42.5% (used 870 bytes from 2048 bytes)
Flash: [=== ] 30.6% (used 5013 bytes from 16384 bytes)
Configuring upload protocol...
AVAILABLE: custom
CURRENT: upload_protocol = custom
Uploading .pio\build\ATtiny1616\firmware.hex
Then the avrdude errors detailed above, then the upload:
avrdude: AVR device initialized and ready to accept instructions
avrdude: device signature = 0x1e9421 (probably t1616)
avrdude: erasing chip
avrdude: reading input file .pio\build\ATtiny1616\firmware.hex for flash
with 5013 bytes in 1 section within [0, 0x1394]
using 79 pages and 43 pad bytes
avrdude: writing 5013 bytes flash ...
Writing | ################################################## | 100% 4.59s
avrdude: 5013 bytes of flash written
avrdude: verifying flash memory against .pio\build\ATtiny1616\firmware.hex
Reading | ################################################## | 100% 4.53s
avrdude: 5013 bytes of flash verified
avrdude done. Thank you.
I’ve used a couple of .ELF file viewers to try to work out what might be different between the two compile outputs, but quickly get in over my head 
I’d appreciate any pointers
