[Arm-netbook] IC for analog and digital buttons (EOMA-68)

[Luke Kenneth Casson Leighton]

On Wed, Aug 6, 2014 at 6:10 PM, Daniel Iglesias
<daniel.iglesias at gmail.com> wrote:
> 06/08/14 14:18, Luke Kenneth Casson Leighton wrote:
>>
>> On Wed, Aug 6, 2014 at 1:19 PM, Miguel Garcia <gacuest at gmail.com> wrote:
>>
>>
>>> A basic sketch of the block diagram:
>>> http://oi57.tinypic.com/16aeffb.jpg
>>
>>   ok great.
>>
>>   so, things that are missing from the diagram:
>>
>>   * digital GPIO IC connects its IRQ-OUT to EINT0-IN on EOMA68
>
> What's the purpose of this IRQ line? After looking at the GPIO IC datasheet
> I'm assuming it's there so the IC is polled only when the inputs change, is
> this correct? (i.e., avoiding busy-waiting)

 http://www.nxp.com/products/interface_and_connectivity/i2c/i2c_general_purpose_i_o/series/PCA9575.html

 you _want_ one that has an IRQ.  don't use one that doesn't.

>
>>   * one GPIO IC IN pin is needed for MicroSD "detect"
>>   * EOMA68 PWM out goes to LCD PWM
>>   * digital GPIO IC OUT pin needed for power-up of LCD
>>   * accelerometer IRQ goes to digital GPIO IC IN
>>   * digital GPIO IC IN pin connects to IRQ-OUT of AXP209
>>   * digital GPIO IC IN pin connects to Headphone-detect (in case you
>> want to alter volume on headphone-out)
>>
>> so you probably want at least 2 16-pin Digital GPIO ICs because that's
>> quite a lot of GPIO.  you have two EINTs so you can at least chain all
>> the EINT IRQs together.
>>
>> if you start using USB (with an STM32F) then you will need 2 USB hubs
>> (or drop the USB Flash Drive idea)
>>
>> if you use UART (with an STM32F) i would be concerned about the speed
>> / latency of the control protocol communicating data.
>
> Low latency is crucial, otherwise we'd just buy a random Android console
> from JXD instead of building our own one, so thanks for bringing that up :)
> A polling rate higher than 100 Hz would be nice. Assuming 6 8-bit ADC's, 19
> digital buttons and a 200 Hz polling rate that'd give 13400 bps plus
> overhead, which isn't too high. How much latency do you expect there would
> be, depending on whether we use UART or SPI?

 SPI goes up to 25mhz (i believe), UART you will be very lucky to get
4mhz if you are extremely careful, you can realistically expect 115200
(1mhz).