Recommended Connector and Wiring Diagram for ATtiny84A In-System Programming via SPI Port

I recently designed a circuit using an ATtiny84A and found it difficult to locate information from Microchip Technology regarding connector pinouts, wiring diagrams, and connector part numbers for In-System Programming (ISP) via the Serial Peripheral Interface (SPI) using an Atmel-ICE programmer.

This article presents a working circuit, design description, and bill of materials (BOM) for others to reference when creating their own ATtiny84A-based circuits to save time and guarantee ISP functionality.

Figure 1: Schematic for ATtiny84A ISP via SPI using an Atmel-ICE programmer

Figure 1 shows a verified working schematic of an ATtiny84A microcontroller and in-system programming connector that are compatible with Atmel-ICE programmers. The circuit uses a 20 kohm pull-up resistor on the RESETn line to increase the strength of its default internal weak pull-up resistor, which can be between 30 kohm and 60 kohms according to the datasheet. Without the 20 kohm external pull-up resistor, the microcontroller could be susceptible to false resets from electrical noise on the RESETn line. Note: some experts recommend a 10 kohm external pull-up resistor, and others recommend no resistor at all. The 20 kohm resistor is a good middle ground that strengthens the RESETn's pull-up resistance while keeping overall current consumption of the device and circuit low.

Figure 2: Pinout of ISP connector compatible with Atmel-ICE programmer

Figure 2 shows the physical implementation of the schematic from Figure 1. It details the pinout of the 6-pin connector that mates with the Atmel-ICE programmer, shown in Figure 3 below.

Figure 3: Atmel-ICE programmer

Figure 4: Factory default fuse settings of ATtiny84A in Microchip Studio

Figure 4 shows the default ATtiny84A fuse settings within Microchip Studio Integrated Development Environment (IDE). The two important fuses for ISP are RSTDISBL (reset disable) and SPIEN (enable serial programming). The ATtiny84A comes from the factory with settings for RSTDISBL and SPIEN that allow for ISP via SPI by default, and no changes to fuses are required. The image is shown for reference, only.

The table below shows parts used in this circuit.

PART DESCRIPTION	MANUFACTURER	MPN
Programmer for SAM and AVR microcontrollers	Microchip Technology	ATATMEL-ICE-BASIC
8-bit AVR microcontroller with 8 KB flash, 14-DIP	Microchip Technology	ATTINY84A-PU
Connector, header, through hole, 6-position 0.100" (2.54mm)	Harting	09185067324
20 kohm, ±1%, 1/6W, through hole resistor, axial metal film	Yageo	MFR-12FTF52-20K