From 1eaee8ae3aab17ba6d56e09c0e2c7a4072414127 Mon Sep 17 00:00:00 2001 From: Nicholas Chin Date: Fri, 7 Apr 2023 18:12:14 -0600 Subject: [PATCH] Add information about RPi drive strength --- site/docs/install/spi.md | 58 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) diff --git a/site/docs/install/spi.md b/site/docs/install/spi.md index 71f820e..e927ec7 100644 --- a/site/docs/install/spi.md +++ b/site/docs/install/spi.md @@ -246,6 +246,64 @@ Under the Interface section, you can enable SPI. The device for communicating via SPI as at `/dev/spidev0.0` +RPi Drive Strength +------------------ +Flashrom on the RPi may not be able to detect the SPI flash chip on some +systems, even if your wiring and clip are set up perfectly. This may be due to +the drive strength of the Raspberry Pi GPIOs, which is 8mA by default. Drive +strength is essentially the maximum current the pin can output while also +maintaining the minimum high-level voltage. In the case of the Pi, this voltage +is 3.0V. + +Similarly, the SPI flash chip has a minimum voltage it will accept as a high +logic value. This is commonly 0.7\*VCC for SPI flash, which is 2.31V for 3.3V +chips. If the drive strength is too low, the voltage at the pins of the flash +chip may fall below this minimum voltage, causing it to register as a low logic +value instead of the high value that was sent. + +On many systems, the VCC pin of the SPI flash is shared with other chips on the +system, causing them to be powered through the voltage supplied through your +programming clip. In this case, parts of the chipset may power up, and it may +attempt to set the SPI lines high or low, interfering with the data the Pi is +trying to send. If the Pi and chipset are trying to set a pin to different +values, the side with a greater drive strength will be able to "pull" the +voltage toward the level it wants to set. + +Fortunately, the drive strength of the Raspberry Pi can be increased up to +16mA. There are a few tools that can set this, such as the pigs utility from +the pigpio project. On the Raspberry Pi OS, the following commands should +install pigpio and set the drive strength to 16mA: + +Install pigpio: + + sudo apt install pigpio + +Start the pigpiod daemon, which the pigs utility communicates with to interact +with the gpios: + + sudo pigpiod + +Set the drive strength of GPIO group 0, which contains the spi0 pins, to 16mA: + + pigs pads 0 16 + +Note that the drive strength hardware works in multiples of 2mA, and pigs will +round odd values up to the next multiple of 2. You can check the current drive +strength using + + pigs padg 0 + +WARNING: If the chipset is very strongly trying to drive a pin to a value +opposite that of the Pi, more than 16mA pass through the Pi's GPIO pins, which +may damage them as they are only designed for 16mA. The drive strength is NOT a +current limit. That said, this is a risk to the Pi regardless of the drive +strength. Resistors between the chipset and the flash should protect against +this, though not all boards have these. + +See + +for more information about the drive strength control on the Pi. + Caution about RPi -----------------