Longan Nano : I2C Master

 After SPI, serial and Gpio, the next building block is I2C/Master.

That one is a bit peculiar.





Overview/DMA

It is actually a finite state machine. 

Each transition of a I2C transmission is associated with  an event, with a corresponding "event" interrupt.

The sequence is StartSent-AddressSent-DataDataData-BTC-Stop

Simple enough.

Where it becomes tricky is when you want to use DMA with it. You have to manage two interrupts source in parallel. It ends up as  : 

1- Do the beginning as usual, using  plain I2C interrupt events.

2- When reaching the "data" stage, start a DMA transfer + disable I2C event interrupt

(we trigger the DMA transfer only if the # of data is > 2, else we do it using basic interrupts)

3- When the DMA stage is done, re-enable the I2C event interrupts  to finish the transfer


To give an example:  to refresh the SSD1306 screen in one step (1024 bytes), we only need ~ 6 interrupts ( 5 I2C + 1 DMA) rather than ~ 1030 interrupts in plain i2c interrupt mode.


I did not find the event attached to the Stop condition, so it is done in polling mode at the moment.

Multiwrite

There is a second change compared to the simple Arduino Api : Multiwrite

You can send a list of data chunks as a single transfer.

That is very helpful to avoid copying data just to add a header.

SSD1306 example : When updating the screen with the SSD1306, you have to add a command byte in front of the raw data. That makes using DMA difficult and/or lead to duplicating the data.

Using the multiwrite, we'll just do a multiwrite made of 2 chunks : The command byte then the screen internal buffer.  That will be sent inside a single transfer. No need to duplicate data, we can send the whole buffer in one go.

Other Problem

A more general problem was spotted : DMA channels.

The DMA channels are shared among several peripherals, each of them is hardwired to a few peripherals usage.

So far it is more or less statically used. When you use SPI0,  the underlying dma channels are automatically initialized and used. 

The problem arises when using too many peripherals :  they will collide in terms of DMA channels.

It should be a more dynamic process, where the DMA channels  are allocated on the fly, only upon usage (with the associated mutex), so that they can coexist peacefully.


The mandatory SSD1306 demo pic : 







Comments

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=...
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt...
Read more