Problem with ISP on micro:bit

Hi Enrico

I have used SPI with the micro:bit a couple of times - with two different 7-seg displays. I can't see from your project what SPI controller IC you are trying to control. If you could post the datasheet for that component it would be helpful.  In addition to the SPI controls, there is usually another pin called chip select or CS on the controller IC to "latch" the data to the display. To send anything to the display you need to take the chip select pin (P0) low, then send the data, then take the chip select pin high again.Here is my micropython code for a MAX7219 controller.

from microbit import *

# setup SPI

spi.init(baudrate=1000000, bits=8, mode=0, sclk=pin13, mosi=pin15, miso=pin14)

# initialisation code

pin0.write_digital(0)

spi.write(b'\x0f\x00')  # enable normal mode and disable test mode

pin0.write_digital(1) # latch data

sleep(300)

With best wishes

Steve

Hi Enrico

I have used SPI with the micro:bit a couple of times - with two different 7-seg displays. I can't see from your project what SPI controller IC you are trying to control. If you could post the datasheet for that component it would be helpful.  In addition to the SPI controls, there is usually another pin called chip select or CS on the controller IC to "latch" the data to the display. To send anything to the display you need to take the chip select pin (P0) low, then send the data, then take the chip select pin high again.Here is my micropython code for a MAX7219 controller.

from microbit import *

# setup SPI

spi.init(baudrate=1000000, bits=8, mode=0, sclk=pin13, mosi=pin15, miso=pin14)

# initialisation code

pin0.write_digital(0)

spi.write(b'\x0f\x00')  # enable normal mode and disable test mode

pin0.write_digital(1) # latch data

sleep(300)

With best wishes

Steve

Hello Steve,

thank you very much for your suggestions. I will try immediately. You mention pin0. As far as I know, there is no difference between pin0 or any other digital I/O pin or for some reason that I don't know it is absolutely necessary to use this pin? The reason is that I will use pin0 to generate audio frequency and as far as I know only pin0 can do this easily in the micro:bit.

I am aware of the CS pin (number 12 on the 7219) asI am using the same IC. Now I will test your suggested program and see what happens with pin0. I will update you shortly.

Enrico

Sorry, I chose P0 at random as the CS pin (you can use any unused pin). In your case, just modify my code but connect CS  on the 7219 to (say) P12 on the micro:bit and change my code to:

pin12.write_digital(0) etc etc.

The MAX7219 is quite complicated to get fully working (it has many "modes") but my code snippet will get "test mode" working easily. I have a blog post here that goes into more detail about using this controller with the micro:bit.

Good luck.

Steve

Hello Steve,

Thank you for your support. Now it wkrkw and tomktrow I will setup the software.

Enrico

Hi Enrico

That's great news! "Test mode" looks good   I look forward to following your project to completion.

With best wishes

Steve

Hi Steve,

thank you again. I took a look at the link on your blog and it seems very complete and detailed, I think it is very useful for my project.

Cheers, Enrico

Let me know if you have any problems. I should warn you that loading my whole python program will take you very close to the memory limit of the micro:bit, so you should just take the bits you need.

In particular, if you only need numerals, you can delete a lot of the "dictionary" (e.g. lines 54 to 105). Also, once you have it working, you won't need "test mode" any more (lines 5 to 8). In normal use you won't need lines 10 to 16 (starting "# zero-out all registers"). This function allocates a large array which uses a lot of memory. Unless you delete these parts, you probably won't have enough memory for your other code.

Obviously you'll have to change the "scan limit" register (0x0b) to 0x03 (4 columns) instead of 0x07 (8 columns).

Otherwise I think you should be OK. Getting that display working fully took me a lot of effort so I hope I've saved you some of that!

With best wishes

Steve

Hello Steve, I think this is a very useful thread ...

Thank you for the suggestion, I was just having in mind to extract just the essential parts from your job, due the supposed problems of memory. Your details are useful to avoid to delete needed parts and vice-versa.

My 2 cent suggestion instead is on how Python import works. I see that a common practice is always using from micro:bit import * but in this way you include ALL the available classes. Most of them are totally useless in many situations so a lot of memory can be saved importing only the right library(es) For example, if you only need SPI it is not the worth to import everything else but only the following :

from microbit import SPI PIN0 PIN13 PIN14 PIN15

As far as I know this can save a lot of memory.

I am not an expert in mycroPython so at the moment what I am investigating is the exact name of the microbit libraries (to choose what import and what don't) and how much memory the parts occupy.

Enrico

You are quite right. I find it easier to just import * when I am developing a program. Then I should optimise it later by only importing the modules that I need, once I know which pins or other built-in modules that I am using (e.g. SPI, I2C). However, I usually forget to do this! By default Mu also downloads comments  and whitespace to the micro:bit (which takes up more memory) but you can turn this off  ("minify") in the latest version of Mu. See here.