revisiting the 434 MHz band

Werner Almesberger werner at
Thu Jul 22 06:41:39 EDT 2010

Wolfgang Spraul wrote:
> did you mention recently that you found SPI test points on the PCB behind
> the LCM? If so we should definitely document that.

Ah, I just found them in the schematics ;-) But there are two problems:

1) it's only one direction. You have CS (= SS), SCL (= SCLK), and SDA
(= MOSI). Never mind that they're named as if this was I2C. The LCD
data sheet shows it's just good old SPI. MISO is missing. Also, to
connect another peripheral, you would need another slave select line.
The RF modules/chips generally also need at least one more signal to
signal an interrupt, and some have additional signals.

There are no unused lines on the FPC connecting the display to the
main PCB, although it's probably safe to reassign some of the +V3.3
and GND lines in a future PCB revision. (There's 4 x +V3.3 and 6 x

2) These SPI signals connect to regular GPIOs on the CPU, not the SPI
port. So it's not as efficient as it could be. This is not an issue
for the few configuration bytes sent to an LCM, but it might be for
high-speed communication.

> Do you think we can hookup HopeRF modules to those test points?

I was indeed thinking of connecting an RF module there. There's
plenty of room on the left side of the LCM and the display case would
be a good place for an antenna. You'd still have a large piece of
grounded metal nearby (the LCM itself), but anywhere else it's likely
to be worse.

- Werner

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