Hi,
I have a Zynq device which manages the configuration of a Spartan 7 FPGA through the Slave Serial Interface. The CCLK_0 and DIN pins of the FPGA are connected to the sclk and MOSI pins of the SPI1 bus, directly managed by the PS of the Zynq.
At the beginning I had some issues, which, after several tests, led me to the conclusion that the CCLK_0 pin of the Spartan7 has an internal weak pull-up (of about 15kohm). Indeed, if I disable the SPI controller of the Zynq device, I see the clock line go high, and it goes down again as soon as I enable the controller. I performed the very same test without connecting the clock line to the CCLK_0 pin and this time the voltage level is always zero, so the problem is not related to the Zynq PS, but to the Spartan 7. I think that when the SPI controller is disabled, the Zynq leaves those pins 3-stated and when it is enabled it directly drives them. Anyway, the problem is not with the Zynq, since the behaviour is different with or without the Spartan 7 connected.
By connecting a pull-down resistor in parallel, I managed to configure the FPGA and I realized that the pull-up resistance should be in the order of 15 kohm. I read UG 470 (https://www.xilinx.com/support/documentation/user_guides/ug470_7Series_Config.pdf) and in the Overview section I found:
- In Master configuration modes, the 7 series device drives CCLK from an internal oscillator. To select the desired frequency, the bitstream -g ConfigRate option is used. The BitGen section of UG628, ISE Command Line Tools User Guide provides more information for the ISE Design Suite. The Device Configuration Bitstream Settings section of UG908, Vivado Programming and Debugging User Guide provides more information for the Vivado Design Suite. After configuration, the CCLK is turned OFF unless the persist option is selected or SEU detection is used. See Persist Option in Chapter 6. The CCLK pin is 3-stated with a weak pull-up.
- In Slave configuration modes, CCLK is an input.
Even if the Spartan 7 is in Slave configuration mode (M[2:0] = '111' SlaveSerial) and CCLK_0 should be an input, I think this pin has an internal weak pull-up (about 15 kohm). This could explain all the thing that I saw.
Can anyone tell me if this is true?
