EN_ACTIVE_CLK and DIS_ACTIVE_CLK enable you to override the internal oscillator during the active configuration scheme.
The AS configuration controllers use the internal oscillator as the clock source. You can change the clock source to CLKUSR through the JTAG instruction.
The EN_ACTIVE_CLK and DIS_ACTIVE_CLK JTAG instructions toggle on or off whether or not the active clock is sourced from the CLKUSR pin or the internal configuration oscillator. To source the active clock from the CLKUSR pin, issue the EN_ACTIVE_CLK instruction. The CLKUSR pin becomes the active clock source.
When using the EN_ACTIVE_CLK
instruction, you must enable the internal oscillator for the clock change to occur. By default, the configuration oscillator is disabled after configuration and initialization is complete as well as the device has entered user mode. However, the internal oscillator is enabled in user mode by any of the following conditions:
- A reconfiguration event (e.g. driving the nCONFIG pin to go low)
- Remote update is enabled
- Error detection is enabled
Note: When using the EN_ACTIVE_CLK and DIS_ACTIVE_CLK JTAG instructions to override the internal oscillator, you must clock the CLKUSR pin at two times the expected DCLK frequency. The CLKUSR pin allows a maximum frequency of 40 MHz (40 MHz DCLK).
Normally, a test instrument uses the CLKUSR
pin when it wants to drive its own clock to control the AS state machine. To revert the clock source back to the configuration oscillator, issue the DIS_ACTIVE_CLK
instruction. After you issue the DIS_ACTIVE_CLK
instruction, you must continue to clock the CLKUSR
pin for 10 clock cycles. Otherwise, even toggling the nCONFIG
pin does not revert the clock source and reconfiguration does not occur. A POR reverts the clock source back to the configuration oscillator. Toggling the nCONFIG
pin or driving the JTAG state machine to reset state does not revert the clock source.