The time it takes for DRDY to go high again after a command
depends on the command. Following is a list of commands and
the time required to process them and then raise DRDY:
3 Serial Communications
These devices use SPI communications, in slave mode.
The host device always initiates communications sequences;
the QT is incapable of chattering data back to the host. This is
intentional for FMEA purposes so that the host always has total
control over the communications with the QT60xx8. In SPI
mode the device is a slave, so that even return data following a
command is controlled by the host.
0x0E Eeprom CRC
0x01 Load Setups
All other commands:
[ 25ms
[ 25ms
[ 2ms between bytes;
[ 40µs after CRC byte is sent
Other DRDY specs:
Min time DRDY is low: 1µs
Min time DRDY is low
A command from the host always ends in a response of some
kind from the QT. Some transmission types from the host or the
QT employ a CRC check byte to provide for robust
communications.
after reset:
1ms
3.2 SPI Communications
A DRDY line is provided that handshakes transmissions.
Generally this is needed by the host from the QT to ensure that
transmissions are not sent when the QT is busy or has not yet
processed a prior command.
SPI communications operates in slave mode only, and obeys
DRDY control signaling. The clocking is as follows:
Clock idle:
High
Clock shift out edge:
Clock data in edge:
Max clock rate:
Falling
Rising
1.5MHz
Initiating or Resetting Communications: After a reset, or,
should communications be lost due to noise or out-of-sequence
reception, the host should send a 0x0f (return last command)
command repeatedly until the compliment of 0x0f, i.e. 0xf0, is
received back. Then, the host can resume normal run mode
communications from a clean start.
SPI mode requires 5 signals to operate:
MOSI - Master out / Slave in data pin; used as an input for
data from the host (master). This pin should be connected
to the MOSI (DO) pin of the host device.
Poll rate: The typical poll rate in normal ‘run’ operation should
be no faster than once per 10ms; 25ms is more than fast
enough to extract status data using the 0x06 command (report
first key: see page 13) in most situations. Streaming multi-byte
response commands like the 0x0d command (dump setups: see
page 13) or multi-byte response commands like 0x07 can and
should pace at the maximum possible rate.
MISO - Master in / Slave out data pin; used as an output for
data to the host. This pin should be connected to the MISO
Figure 3-1 Basic SPI Connections
Run Poll Sequence: In normal run mode the host should limit
traffic with a minimalist control structure (see also Section 4.18).
The host should just send a 0x06 command until something
requires a deeper state inspection. If there is more than one key
in detect, the host should use 0x07 to find which additional keys
are in detect. If there is an error, the host should ascertain the
error type based on commands 0x0b and 0x0c and take
appropriate action. Issuing a 0x07 command all the time is
wasteful of bandwidth, requires more host processor time, and
actually conveys less information (no error flags are sent via a
0x07 command).
Host MCU
QT60xx8
P_IN
P_OUT
SCK
MISO
MOSI
DRDY
SS
SCK
MISO
MOSI
Figure 3-2 Filtered SPI Connections
3.1 DRDY Pin
DRDY is an open-drain output with an internal 20K ~ 50K pullup
resistor.
Host MCU
QT60xx8 Circuit
X drives
P_IN
P_OUT1
SCK
DRDY
Ra
Ra
Ra
Ra
Serial communications pacing is controlled by this pin. The host
is permitted to send data only when DRDY is high. After a byte
is received DRDY will always go low even if only for a few
microseconds; during this period the host should not send data.
Therefore, after each byte transmission the host should first
check that DRDY is high again.
(1 of 8
Ca
Ca
1K
1K
Xn
Yn
shown)
SS
Ca
Ca
SCK
MISO
MISO
MOSI
Y Lines
(1 of 3
shown)
If the host desires to send a byte to the QT it should behave as
follows:
MOSI
Ra
1K
Ca
1. If DRDY is low, wait
P_OUT2
RESET
2. If DRDY is high: send a command to QT
3. Wait at least 40µs (time S5 in Figure 3-3: DRDY is
guaranteed to go low before this 40µs expires)
4. Wait until DRDY is high (it may already be high again)
5. Send next command or a null byte 0x00 to QT
1nF
Recommended Values of Ra & Ca
SPI Clock Rate
Ra
Ca
1.5MHz
680
100pF
270pF
470pF
1nF
400kHz
1,000
2,200
2,200
100kHz
50kHz
lQ
10
QT60248-AS R4.02/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
