Monday, June 1, 2015

AVR ISP Pi Cap user guide

This is the permanent home for the user guide for the AVR ISP Pi Cap.

This board provides level shifting and voltage protection (including hot-plugging) for AVR programming via the GPIO pins on a Raspberry Pi. It allows you to select the target power supplied to the target as either 3.3 volts, 5 volts or no power (meaning that the target is self-powered). The programming pins for the target will be level-shifted to whatever voltage is supplied to or by the target when programming is taking place, and will be high impedance otherwise. The buffer chip protects the Pi's GPIO pins from any invalid excursions. An AP2331 hot-plug protection chip protects the Pi from shorts and hot-plugging voltage sags.

The board comes with the stacking header (for the Pi) and the shrouded DIP header (for the target) uninstalled. It is strongly recommended that you install the stacking header first so that the shrouded DIP header doesn't get in the way of your soldering iron.

The GPIO pins selected are the MOSI/MISO/SCK pins for SPI port 0, however !RESET is connected to GPIO pin 25 rather than using one of the SPI CE pins. This allows !RESET to be asserted in a separate operation rather than being asserted and deasserted with every SPI operation performed during a single programming session. With this configuration, you have a choice of using the "linuxgpio" driver built-in to avrdude version 6.1, or the higher performance native SPI driver.

To use bit-banged GPIO, configure AVRDUDE for the "linuxgpio" programmer. Add this section to your avrdude.conf file:

    id = "pihat";
    desc = "Use the Linux sysfs interface to bitbang GPIO lines";
    type = "linuxgpio";
    reset = 25;
    miso = 9;
    mosi = 10;
    sck = 11;

Having done that, you should be able to use the -c pihat argument to avrdude to program a target. If it doesn't work, make sure you have an avrdude binary compiled to include the "linuxgpio" programming driver. AdaFruit has a guide that shows how to build avrdude 6.1 to include that driver. The LED will light when the !RESET gpio pin is brought low, and the bus buffer chip will bring the target !RESET pin low and assert the ISP signals on their relative pins.

Note that as of 6.1, there is an issue with the linuxgpio driver in avrdude. There is a race condition between avrdude writing to the GPIO "export" node to export the GPIO pins, and udev setting the permissions properly on the newly created nodes. The only workaround (at present) is to patch avrdude to add a delay between the export and the rest of the operations. Here's a patch.

If you are used to using -B to slow down the SPI clock, you can still do so, but the correct argument is -i instead (with the same numeric value).

To instead use the SPI driver, you'll need to apply a patch to avrdude to add the "linuxspi" driver (TBD).

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