MCP2502X datasheet(49/66 Pages) MICROCHIP

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DS21664D-page 49

MCP2502X/5X

FIGURE 8-3:

SIMPLIFIED BLOCK DIAGRAM OF ON-CHIP RESET CIRCUIT

8.4.2

POWER-UP TIMER

The Power-up Timer (PWRT) provides a fixed, 72 ms

nominal time-out, on power-up only, from the POR. The

Power-up Timer operates on an internal RC oscillator,

with the device being kept in reset as long as the PWRT

is active. The PWRT's time delay allows VDD to rise to

an acceptable level. The power-up time delay will vary

from device to device due to VDD, temperature and

process variation. For more information, please see

Section 9.2 “DC Characteristics”.

8.5

Oscillator Start-up Timer

The Oscillator Start-up Timer (OST) provides a 512

oscillator cycle (TOSC) delay after the PWRT delay is

complete. This ensures that the crystal oscillator has

started and stabilized and must be less than the total

time it takes (704 oscillator cycles or 44 TQ) for the

minimum standard data frame or remote transmit

message to be completed on the CAN bus once a

wake-up from SLEEP occurs. The OST time-out is

invoked only on Power-on Reset or wake-up from

SLEEP.

8.6

Power-down Mode (SLEEP)

Power-down mode (or SLEEP) is enabled via the

SLPEN bit in the OPTREG2 register. When enabled,

the MCP2502X/5X will enter SLEEP once the CAN bus

has been idle for a minimum 1408 bit times while in

Normal mode.

Additionally, the device may be configured to enter

SLEEP while in Listen-only mode immediately after

power-up if there is no activity on the CAN bus.

Subsequent CAN bus activity will wake the device up

from SLEEP and the NEXT message will be confirmed

as a valid message before entering Normal mode. This

feature is enabled via the PUSLP bit in the OPTREG2

While in SLEEP, the I/O ports maintain the status they

had before the SLEEP instruction was executed

(driving high, low or hi-impedance).

The following operations will not function while the

device is in SLEEP:

• A/D Module data conversion

• Auto-conversion mode

• Auto-messaging

• PWM module and outputs

• Clock output

8.6.1

WAKE-UP FROM SLEEP

The MCP2502X/5X can wake-up from SLEEP through

one of the following events:

• External reset input on RST pin

• Transmit-on-change due to edge detected on

GPIO pin

• Activity detected on CAN bus

For the device to wake-up due to a GPIO transmit-on-

change, the corresponding interrupt enable bit must be

set (enabled). Wake-up occurs regardless of the state

of the GIE bit.

If a wake-up from SLEEP is caused by activity on the

CAN bus, the message that caused the wake-up will not

be received or acknowledged by the MCP2502X/5X.

RST

VDD

OSC1

VDD Rise

Detect

Power-on Reset

OST

10-bit Ripple Counter

PWRT

On-chip

RC OSC

Enable PWRT

Enable OST

Chip Reset

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