16-bit SPI trying to read from 22-clock cycle ADC
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From: ChrisQ on 8 Sep 2009 11:47 Bill wrote: > Hi, > > I'm trying to pull out data from the ADS8320 (a 16-bit ADC by Analog > Devices. See bottom of page 10 in > http://focus.ti.com/lit/ds/symlink/ads8320.pdf ) using the SPI in an > AT91SAM7S256. The problem is that the ADC needs 6 extra clock cycles > to sample the analog signal, before the 16 cycles that will output > each of the conversion result bits. So, a complete ADC cycle involves > a minimum of 22 clock cycles. Now, the SPI in the AT91SAM7 (and, as > far as I've seen, in all other MCUs), cannot generate more than 16 > clock cycles within one CS activation. > > How am I supposed to do this, in an elegant way? Of course I could bit > bang those lines, but I hate doing that, because it adds load to the > CPU, and doesn't take advantage of the SPI and DMA. > > The AT91SAM7S256 allows holding the CS low until a new device is > addressed, so I could initiate two 11-bit readings in a row (in such a > way that the ADC would think it is a single CS assertion with 22 clock > cycles inside), and discard the bits with no information, but that's > still ugly to me. It would use the SPI, but not the DMA, and the two > readings would be different (the first one should hold CS low. The > second one should leave it high), which is kind of non-homogeneous. > > Any more elegant ideas? > > Thank you. Hi, I wrote a driver for the (similar ?) ad7705. It was driven by a int timer based state machine to sequence start of conversion, read result etc, over the 2 input channels. This was on a Dragonball (68k) and iirc, the spi port may have had a short queue to make life easier. There was no cs on the spi port, so an io port line was used for chip select. I do remember that the ads device was a little weird in terms of programming, and there are some 'hinted' at issues in the data sheet, but the results were fine. It was 2004, so would need to dig out the code, but would assume the pragmatic approach was taken. ie: just wrote / read as many bytes as were required to get at the device and masked off the unwanted stuff. It's far from an inelegant solution if implemented and documented properly... Regards, Chris
From: langwadt on 8 Sep 2009 12:38 On 8 Sep., 17:23, Bill <a...(a)a.a> wrote: > On Tue, 08 Sep 2009 09:11:13 -0500, Vladimir Vassilevsky > > <nos...(a)nowhere.com> wrote: > >> I'm trying to pull out data from the ADS8320 (a 16-bit ADC by Analog > >> Devices. > >^^^^^^^^^^^^^^^^^^^^^^ > > >> See bottom of page 10 in > >>http://focus.ti.com/lit/ds/symlink/ads8320.pdf) > >^^^^^^^^^^^^^^^^^^^^^ > > >LOL > > :-) Sorry. TI. I'm using other Analog Devices ICs. > > >Generate transactions by SPI, but control the CS signal as GPIO. > >I.e. assert the CS, read the required number of bits by SPI, deassert > >the CS. > > But still, bye bye DMA. > > I'm very curious... what were the guys at TI (and at some other > companies) thinking when they designed this ADC with SPI interface? > Which SPI hardware were they thinking would be able to pull out data > in one transaction? Does anyone know of an MCU with SPI hardware able > to do this? Amazing. > > Best, don't remember seeing much of anything spi especially DAC/ADCS that didn't require all kinds of trick like bit banging CE, dummy data, extra clocks etc.. guess it is a combination of analog guys doing digital design trying to make it as small as possible with out thinking of what it will talk to and digital guys doing SPI interfaces not realising all the special cases that exist in the real world. anyhoo, have a look at the SSC instead and see if it doesn't fit your needs better than the SPI. -Lasse
From: Mel on 8 Sep 2009 13:06 Bill wrote: > On Tue, 08 Sep 2009 11:23:38 -0400, Mel <mwilson(a)the-wire.com> wrote: > > >>You have Peripheral DMA Controller, use it. > > I would gladly use it, but I don't think it is possible. DMA is not > able neither to automatically change CSAAT (Chip Select Active After > Transfer) configuration between 8-bit transactions 2 and 3, nor to bit > bang CS to my needs. From what I'm seeing here, when the PDC is in charge, the SPI doesn't work in 8-bit transactions. It works in a single transaction involving as many 8-bit bytes as the PDC demands. I see /CS go low, and while it's low 24 clock pulses come out in a single burst, then /CS goes high and life goes on. (This is with the code I posted. Thank the deity for digital storage scopes.) Mel.
From: David Brown on 8 Sep 2009 16:53 Bill wrote: > On Tue, 08 Sep 2009 09:11:13 -0500, Vladimir Vassilevsky > <nospam(a)nowhere.com> wrote: > > >>> I'm trying to pull out data from the ADS8320 (a 16-bit ADC by Analog >>> Devices. >> ^^^^^^^^^^^^^^^^^^^^^^ >> >>> See bottom of page 10 in >>> http://focus.ti.com/lit/ds/symlink/ads8320.pdf ) >> ^^^^^^^^^^^^^^^^^^^^^ >> >> LOL > > :-) Sorry. TI. I'm using other Analog Devices ICs. > >> Generate transactions by SPI, but control the CS signal as GPIO. >> I.e. assert the CS, read the required number of bits by SPI, deassert >> the CS. > > But still, bye bye DMA. > > > I'm very curious... what were the guys at TI (and at some other > companies) thinking when they designed this ADC with SPI interface? > Which SPI hardware were they thinking would be able to pull out data > in one transaction? Does anyone know of an MCU with SPI hardware able > to do this? Amazing. > Freescale microcontrollers with queued SPI peripherals have no problems doing long SPI transfers. Each individual transfer is from 8 to 16 clocks, but you can specify that the chip select will not be deasserted after the first transfer, so that you can easily get 22 clock transfers. There are many ColdFire's (and older 683xx devices) with a queued SPI, and the latest PPC-based MPC5xxx devices have even more powerful SPI peripherals connected to flexible DMA. I don't know if that's what the guys at TI were thinking about, and I doubt if it really helps you here, but there *are* microcontrollers available that can handle all this in the background by DMA (/if/ you can understand the documentation...).
From: karthikbalaguru on 8 Sep 2009 17:33
On Sep 8, 6:53 pm, Bill <a...(a)a.a> wrote: > Hi, > > I'm trying to pull out data from the ADS8320 (a 16-bit ADC by Analog > Devices. See bottom of page 10 inhttp://focus.ti.com/lit/ds/symlink/ads8320.pdf) using the SPI in an > AT91SAM7S256. The problem is that the ADC needs 6 extra clock cycles > to sample the analog signal, before the 16 cycles that will output > each of the conversion result bits. So, a complete ADC cycle involves > a minimum of 22 clock cycles. Now, the SPI in the AT91SAM7 (and, as > far as I've seen, in all other MCUs), cannot generate more than 16 > clock cycles within one CS activation. > > How am I supposed to do this, in an elegant way? Of course I could bit > bang those lines, but I hate doing that, because it adds load to the > CPU, and doesn't take advantage of the SPI and DMA. > > The AT91SAM7S256 allows holding the CS low until a new device is > addressed, so I could initiate two 11-bit readings in a row (in such a > way that the ADC would think it is a single CS assertion with 22 clock > cycles inside), and discard the bits with no information, but that's > still ugly to me. It would use the SPI, but not the DMA, and the two > readings would be different (the first one should hold CS low. The > second one should leave it high), which is kind of non-homogeneous. > > Any more elegant ideas? > It appears strange. From my experience, normally TI would definitely have some reasons for any such strange things. In this scenario, bit banging appears reasonable. Well, that might increase the CPU load. The best help would be from the TI support. Karthik Balaguru |