On earlier versions of the Portsdown, GPIO Pin 27 was used for the ADF4351 LE and GPIO 28 was used for the Band LSB (D0). On the Raspberry Pi 4, these GPIO pins are reserved for the EEPROM associated with Raspberry Pi "HATs".
In some circumstances, if these pins are connected to other circuitry (such as band switching drivers), the Raspberry Pi 4 can fail to boot with a new SD Card. I discovered this when trying to build a new Portsdown 4 SD Card on a Portsdown that had been modified from being a Portsdown 2020 and still had an ADF4351 and the band switching circuitry connected. Previously built Portsdown SD Cards worked OK, however new cards would not boot.
So if your Portsdown 4 hangs up on the Rainbow Screen during boot, check that you do not have anything connected to GPIO pins 27 and 28.
I will be amending the Portsdown 4 GPIO Pin allocation table (and software) in due course.
Dave, G8GKQ
Portsdown 4 - Don't Connect GPIO Pins 27 and 28
Re: Portsdown 4 - Don't Connect GPIO Pins 27 and 28
I will be implementing the following changes in the next Portsdown 4 software update:
Band D0, which was on GPIO physical pin 28, will instead be output on GPIO pin 32.
The ADF4351 LE output (currently not used, but watch this space) will move from GPIO pin 27 to GPIO pin 33.
This change will leave GPIO pins 27 and 28 with no connection as recommended by the Raspberry Pi foundation.
The issue appeared to be that these pins are used for i2c signalling (with a potential Raspberry Pi HAT) during the early stages of the boot process of a brand new SD Card, and the boot would hang if the load on one of the pins was too high to enable it to reach the Logic high threshold within the i2c signalling specification.
SD Cards that had already been built as Portsdown cards seemed to skip this step and were not affected by the issue.
Please note that this change will not be made on the Portsdown 2020 or any other Portsdown version that uses a Raspberry Pi 3 - it is a Raspberry Pi 4 issue.
Dave, G8GKQ
Band D0, which was on GPIO physical pin 28, will instead be output on GPIO pin 32.
The ADF4351 LE output (currently not used, but watch this space) will move from GPIO pin 27 to GPIO pin 33.
This change will leave GPIO pins 27 and 28 with no connection as recommended by the Raspberry Pi foundation.
The issue appeared to be that these pins are used for i2c signalling (with a potential Raspberry Pi HAT) during the early stages of the boot process of a brand new SD Card, and the boot would hang if the load on one of the pins was too high to enable it to reach the Logic high threshold within the i2c signalling specification.
SD Cards that had already been built as Portsdown cards seemed to skip this step and were not affected by the issue.
Please note that this change will not be made on the Portsdown 2020 or any other Portsdown version that uses a Raspberry Pi 3 - it is a Raspberry Pi 4 issue.
Dave, G8GKQ