Cypress CY7C64013C Manual De Usuario

Document #: 38-08001 Rev. *B

Page 17 of 51

The XTALIN and XTALOUT are the clock pins to the microcontroller. The user can connect an external oscillator or a crystal to

these pins. When using an external crystal, keep PCB traces between the chip leads and crystal as short as possible (less than

2 cm). A 6-MHz fundamental frequency parallel resonant crystal can be connected to these pins to provide a reference frequency

for the internal PLL. The two internal 30-pF load caps appear in series to the external crystal and would be equivalent to a 15-pF

load. Therefore, the crystal must have a required load capacitance of about 15–18 pF. A ceramic resonator does not allow the

microcontroller to meet the timing specifications of full speed USB and therefore a ceramic resonator is not recommended with

these parts.
An external 6-MHz clock can be applied to the XTALIN pin if the XTALOUT pin is left open. Grounding the XTALOUT pin when

driving XTALIN with an oscillator does not work because the internal clock is effectively shorted to ground.

The CY7C64x13C supports two resets: Power-On Reset (POR) and a Watchdog Reset (WDR). Each of these resets causes:

• all registers to be restored to their default states,
• the USB Device Address to be set to 0,
• all interrupts to be disabled,
• the PSP and Data Stack Pointer (DSP) to be set to memory address 0x00.

The occurrence of a reset is recorded in the Processor Status and Control Register, as described in Section 15.0. Bits 4 and 6

are used to record the occurrence of POR and WDR, respectively. Firmware can interrogate these bits to determine the cause

of a reset. 
Program execution starts at ROM address 0x0000 after a reset. Although this looks like interrupt vector 0, there is an important

difference. Reset processing does NOT push the program counter, carry flag, and zero flag onto program stack. The firmware

reset handler should configure the hardware before the “main” loop of code. Attempting to execute a RET or RETI in the firmware

reset handler causes unpredictable execution results.

When V

CC

 is first applied to the chip, the Power-On Reset (POR) signal is asserted and the CY7C64x13C enters a “semi-suspend”

state. During the semi-suspend state, which is different from the suspend state defined in the USB specification, the oscillator

and all other blocks of the part are functional, except for the CPU. This semi-suspend time ensures that both a valid V

CC

 level is

reached and that the internal PLL has time to stabilize before full operation begins. When the V

CC

 has risen above approximately

2.5V, and the oscillator is stable, the POR is deasserted and the on-chip timer starts counting. The first 1 ms of suspend time is

not interruptible, and the semi-suspend state continues for an additional 95 ms unless the count is bypassed by a USB Bus Reset

on the upstream port. The 95 ms provides time for V

CC

 to stabilize at a valid operating voltage before the chip executes code.

If a USB Bus Reset occurs on the upstream port during the 95-ms semi-suspend time, the semi-suspend state is aborted and

program execution begins immediately from address 0x0000. In this case, the Bus Reset interrupt is pending but not serviced

until firmware sets the USB Bus Reset Interrupt Enable bit (bit 0 of register 0x20) and enables interrupts with the EI command. 
The POR signal is asserted whenever V

CC

 drops below approximately 2.5V, and remains asserted until V

CC

 rises above this level

again. Behavior is the same as described above.

The Watchdog Timer Reset (WDR) occurs when the internal Watchdog timer rolls over. Writing any value to the write-only

Watchdog Restart Register at address 0x26 clears the timer. The timer rolls over and WDR occurs if it is not cleared within t

WATCH

(8 ms minimum) of the last clear. Bit 6 of the Processor Status and Control Register is set to record this event (the register contents

are set to 010X0001 by the WDR). A Watchdog Timer Reset lasts for 2 ms, after which the microcontroller begins execution at

ROM address 0x0000.

30 pF 

30 pF